Дисертації з теми "Visual cortical areas"

Visual attention improves sensory processing, as well as perceptual readout and behavior. Over the last decades, many proposals have been put forth to explain how attention affects visual neural processing. These include the modulation of neural firing rates and synchrony, neural tuning properties, and rhythmic, subthreshold activity. Despite the wealth of knowledge provided by previous studies, the way attention shapes interactions between cortical layers within and between visual sensory areas is only just emerging. To investigate this, we studied neural signals from macaque V1 and V4 visual areas, while monkeys performed a covert, feature-based spatial attention task. The data were simultaneously recorded from laminar electrodes disposed normal to cortical surface in both areas (16 contacts, 150 μm inter-contact spacing). Stimuli presentation was based on the overlap of the receptive fields (RFs) of V1 and V4. Channel depths alignment was referenced to laminar layer IV, based on spatial current source density and temporal latency analyses. Our analyses mainly focused on the study of Local Field Potential (LFP) signals, for which we applied local (bipolar) re-referencing offline. We investigated the effects of attention on LFP spectral power and laminar interactions between LFP signals at different depths, both at the local level within V1 and V4, and at the inter-areal level across V1 and V4. Inspired by current progress from literature, we were interested in the characterization of frequency-specific laminar interactions, which we investigated both in terms of rhythmic synchronization by computing spectral coherence, and in terms of directed causal influence, by computing Granger causalities (GCs). The spectral power of LFPs in different frequency bands showed relatively small differences along cortical depths both in V1 and in V4. However, we found attentional effects on LFP spectral power consistent with previous literature. For V1 LFPs, attention to stimuli in RF location mainly resulted in a shift of the low-gamma (∼30-50 Hz) spectral power peak towards (∼3-4 Hz) higher frequencies and increases in power for frequency bands above low-gamma peak frequencies, as well as decreases in power below these frequencies. For V4 LFPs, attention towards stimuli in RF locations caused a decrease in power for frequencies < 20 Hz and a broad band increase for frequencies > 20 Hz. Attention affected spectral coherence within V1 and within V4 layers in similar way as the spectral power modulation described above. Spectral coherence between V1 and V4 channel pairs was increased by attention mainly in the beta band (∼ 15-30 Hz) and the low-gamma range (∼ 30-50 Hz). Attention affected GC interactions in a layer and frequency dependent manner in complex ways, not always compliant with predictions made by the canonical models of laminar feed-forward and feed-back interactions. Within V1, attention increased feed-forward efficacy across almost all low-frequency bands (∼ 2-50 Hz). Within V4, attention mostly increased GCs in the low and high gamma frequency in a 'downwards' direction within the column, i.e. from supragranular to granular and to infragranular layers. Increases were also evident in an ‘upwards’ direction from granular to supragranular layers. For inter-areal GCs, the dominant changes were an increase in the gamma frequency range from V1 granular and infragranular layers to V4 supragranular and granular layers, as well as an increase from V4 supragranular layers to all V1 layers.

Visual attention improves sensory processing, as well as perceptual readout and behavior. Over the last decades, many proposals have been put forth to explain how attention affects visual neural processing. These include the modulation of neural firing rates and synchrony, neural tuning properties, and rhythmic, subthreshold activity. Despite the wealth of knowledge provided by previous studies, the way attention shapes interactions between cortical layers within and between visual sensory areas is only just emerging. To investigate this, we studied neural signals from macaque V1 and V4 visual areas, while monkeys performed a covert, feature-based spatial attention task. The data were simultaneously recorded from laminar electrodes disposed normal to cortical surface in both areas (16 contacts, 150 μm inter-contact spacing). Stimuli presentation was based on the overlap of the receptive fields (RFs) of V1 and V4. Channel depths alignment was referenced to laminar layer IV, based on spatial current source density and temporal latency analyses. Our analyses mainly focused on the study of Local Field Potential (LFP) signals, for which we applied local (bipolar) re-referencing offline. We investigated the effects of attention on LFP spectral power and laminar interactions between LFP signals at different depths, both at the local level within V1 and V4, and at the inter-areal level across V1 and V4. Inspired by current progress from literature, we were interested in the characterization of frequency-specific laminar interactions, which we investigated both in terms of rhythmic synchronization by computing spectral coherence, and in terms of directed causal influence, by computing Granger causalities (GCs). The spectral power of LFPs in different frequency bands showed relatively small differences along cortical depths both in V1 and in V4. However, we found attentional effects on LFP spectral power consistent with previous literature. For V1 LFPs, attention to stimuli in RF location mainly resulted in a shift of the low-gamma (∼30-50 Hz) spectral power peak towards (∼3-4 Hz) higher frequencies and increases in power for frequency bands above low-gamma peak frequencies, as well as decreases in power below these frequencies. For V4 LFPs, attention towards stimuli in RF locations caused a decrease in power for frequencies < 20 Hz and a broad band increase for frequencies > 20 Hz. Attention affected spectral coherence within V1 and within V4 layers in similar way as the spectral power modulation described above. Spectral coherence between V1 and V4 channel pairs was increased by attention mainly in the beta band (∼ 15-30 Hz) and the low-gamma range (∼ 30-50 Hz). Attention affected GC interactions in a layer and frequency dependent manner in complex ways, not always compliant with predictions made by the canonical models of laminar feed-forward and feed-back interactions. Within V1, attention increased feed-forward efficacy across almost all low-frequency bands (∼ 2-50 Hz). Within V4, attention mostly increased GCs in the low and high gamma frequency in a 'downwards' direction within the column, i.e. from supragranular to granular and to infragranular layers. Increases were also evident in an ‘upwards’ direction from granular to supragranular layers. For inter-areal GCs, the dominant changes were an increase in the gamma frequency range from V1 granular and infragranular layers to V4 supragranular and granular layers, as well as an increase from V4 supragranular layers to all V1 layers.

Les patients atteints de maladie de Parkinson présentent des troubles de la marche, parfois paroxystiques, pouvant être aggravés ou améliorés par les stimuli environnementaux. L'attention portée, soit aux stimuli extérieurs, soit à la marche, pourrait ainsi moduler la locomotion.L’objectif principal était donc de mieux caractériser la manière dont les stimuli environnementaux modulent par le biais de réseaux attentionnels la locomotion. Ceci a été étudié chez les sujets sains puis chez les patients parkinsoniens, avec ou sans enrayage cinétique.Nous avons d'abord défini précisément les déficits attentionnels des patients, avec ou sans troubles de la marche. Ils présentaient respectivement des difficultés en flexibilité mentale et plus particulièrement en attention divisée.Nous avons ensuite exploré l'interaction attention-locomotion grâce à l'étude de la préparation motrice. Ainsi, nous avons pu démontrer que les ajustements posturaux anticipés étaient un marqueur sensible de l’attention. Chez les patients, ils pouvaient témoigner d’une altération de l'interaction attention-programmation motrice.L'étude des régions cérébrales activées lors de la locomotion visuo-guidée chez ces patients a permis de confirmer l'implication de structures corticales attentionnelles. Un déséquilibre d’activation au sein du réseau pariéto-prémoteur (nécessaire à la modulation de l'action motrice en fonction des stimuli externes) était présent.Enfin, nous avons essayé de modifier l'excitabilité du cortex prémoteur via des techniques de stimulation magnétique transcrânienne répétitive afin de moduler la locomotion visuo-guidée
Patients with Parkinson 's disease present gait impairments, sometimes sudden and unexpected, either improved or deteriorated with environmental stimuli. Attention focalization, either on external stimuli or on gait, could then modulate locomotion.The main objective was to better characterize how environmental stimuli would modulate locomotion, via attentional networks, in healthy subjects and in parkinsonian patients, with or without freezing of gait.At first, we precisely defined the attentional deficits in patients, with or without gait impairment. They showed altered performance respectively in mental flexibility and in divided attention.Then, we explored the attention-locomotion interaction by studying motor preparation. So, we highlighted that anticipatory postural adjustments were a sensitive marker of attention. In patients, they evidenced an alteration of the attention-motor program interaction.Studying the brain activation during the visuo-driven locomotion in these patients confirmed the involvement of cortical attentional regions. We observed an imbalance inside the parieto-premotor network (useful to modulate motor action according external stimuli)Finally, we tried to change the excitability of the premotor cortex with transcranial magnetic stimulation to modulate visuo-driven locomotion

In this report, we evaluate the role of visual areas responsive to motion in the human brain in the perception of stimulus speed. We first identified and localized V1, V3A, and V5/MT+ in individual participants on the basis of blood oxygenation level-dependent responses obtained in retinotopic mapping experiments and responses to moving gratings. Repetitive transcranial magnetic stimulation (rTMS) was then used to disrupt the normal functioning of the previously localized visual areas in each participant. During the rTMS application, participants were required to perform delayed discrimination of the speed of drifting or spatial frequency of static gratings. The application of rTMS to areas V5/MT and V3A induced a subjective slowing of visual stimuli and (often) caused increases in speed discrimination thresholds. Deficits in spatial frequency discrimination were not observed for applications of rTMS to V3A or V5/MT+. The induced deficits in speed perception were also specific to the cortical site of TMS delivery. The application of TMS to regions of the cortex adjacent to V5/MT and V3A, as well as to area V1, produced no deficits in speed perception. These results suggest that, in addition to area V5/MT+, V3A plays an important role in a cortical network that underpins the perception of stimulus speed in the human brain.

L’emianopsia è un disturbo visivo caratterizzato da cecità in una porzione del campo, controlaterale alla sede di lesione che coinvolge il circuito visivo. Nonostante tale difficoltà, alcune abilità visive residue (“blindsight”) possono essere mantenute nel campo cieco; la probabilità di riscontrare tale fenomeno risulta incrementata dalla presentazione di stimoli in movimento che possono attivare l’area visiva motoria (hMT) senza passare dall’area visiva primaria (V1). Di conseguenza, un comportamento guidato dalla visione risulta possibile nel campo cieco, in assenza di consapevolezza percettiva. Questo progetto di ricerca è costituito da tre sessioni sperimentali svolte con sei pazienti emianoptici e partecipanti sani, allo scopo di esplorare le basi neurali del “blindsight” o della visione residua, valutare la risposta neurale determinata da stimoli presentati nel campo cieco e valutare se lo spostamento dell’attenzione spaziale verso il campo cieco incrementi la risposta sia neurale che comportamentale. Durante la prima sessione è stata valutata la presenza di “blindsight” o di visione residua esaminando la presenza di un punteggio superiore al caso durante lo svolgimento di compiti di discriminazione di movimento e orientamento di stimoli presentati nel campo cieco. In un paziente su quattro (L.F.) si è ottenuto un punteggio superiore al caso in assenza di consapevolezza percettiva nel compito di discriminazione del movimento. In questo caso il punteggio era associato alla sensazione di presentazione dello stimolo riportata dal paziente, che può rimandare al Blindsight di secondo tipo. Nella seconda parte è stata svolta una sessione di neuroimaging (fMRI) utilizzando uno scanner a 3Tesla, allo scopo di i) valutare la presenza di anormalità nella rappresentazione corticale del campo cieco, all’interno della corteccia visiva (Retinotopic Mapping), ii) valutare la posizione e l’attivazione dell’area hMT (hMT Localizer) e iii) valutare la connettività strutturale e l’integrità delle fibre di sostanza bianca nello stesso paziente (Imaging con Tensore di Diffusione, DTI). Nel paziente A.G. abbiamo riscontrato un’ organizzazione retinotopica delle aree visive di basso livello in entrambi gli emisferi, nonostante la lesione interessasse prevalentemente la porzione dorsale della corteccia visiva primaria di sinistra (Retinotopic Mapping); abbiamo osservato l’attivazione dell’area hMT nell’emisfero leso (hMT Localizer) e l’integrità delle vie visive ad eccezione delle radiazioni ottiche nell’ area lesa (DTI). Durante la terza sessione è stato utilizzato un approccio elettrofisiologico. Per ottenere una risposta affidabile presentando stimoli nel campo cieco, è stata utilizzata la tecnica dei potenziali evocati Steady-state (SSVEP) che ha dimostrato essere più informativa rispetto ai potenziali evocati transienti in questo tipo di pazienti. La sessione includeva una stimolazione passiva e un compito di attenzione. L’obiettivo della prima era di valutare la risposta a stimoli che “sfarfallavano” (flickering) ad una frequenza specifica all’interno dei quattro quadranti; è stato osservato che in tutti i pazienti la presentazione dello stimolo nel quadrante cieco produceva una modulazione della risposta neurale che coinvolgeva entrambi gli emisferi. Nel compito di attenzione l’orientamento di quest’ultima verso il campo cieco determinava un incremento della risposta evocata rispetto alla condizione di non attenzione, anche quando quest’ultima veniva rivolta verso il campo cieco, seppur in assenza di consapevolezza percettiva. E’ stata confermata quindi l’utilità degli SSVEP nella valutazione della risposta neurale in seguito alla presentazione di stimoli nel campo cieco. Questi risultati rappresentano un punto chiave interessante per lo studio delle basi neurali della visione inconsapevole in quanto dimostrano come stimoli presentati nel campo cieco possano determinare un’ attività neurale attendibile in varie aree corticali.
Hemianopia is a visual field defect characterized by blindness in the hemifield contralateral to the side of a lesion of the central visual pathway. Despite this loss of vision, it has been shown that some unconscious visual abilities (“blindsight”) might be present in the blind field; the probability of finding this phenomenon can be increased by presenting moving stimuli in the blind field which activate the motion visual area (hMT), bypassing the damaged primary visual area (V1). As a consequence, visually guided behaviour is made possible but perceptual awareness is lacking. The present research project consists of three experimental sessions carried out with six hemianopic patients and healthy participants, in order to explore the neural basis of blindsight or residual vision, to assess whether unseen visual stimuli presented to the blind field can evoke neural responses in the lesioned or intact hemisphere and to evaluate whether shifts of spatial attention to the blind field can enhance these responses as well as the behavioral performance. In the first session we assessed the presence of blindsight or conscious residual vision by testing for the presence of unconscious above chance performance in motion and orientation discrimination tasks with stimuli presented to the blind area. We found evidence of unconscious above chance performance in one patient (L.F.) in the Motion Discrimination Task. In this case the above chance performance was associated with a feeling of something occurring on the screen, reported by the patient that resembles the so-called Blindsight Type II. In the second session we used a neuroimaging technique with the purpose of: i) assess the presence of abnormalities in the cortical representation of the blind visual field in the visual cortex, ii) evaluate position and activation of area hMT and iii) assess the structural connectivity and the integrity of white matter fibers in the same patient. To do that, by using a 3 Tesla Scanner, we carried out a fMRI session with Retinotopic Mapping, hMT Localizer and Diffusion Tensor Imaging procedures (DTI). In patient A.G. we found a retinotopic organization of low-level visual areas in the blind as well as in the intact hemisphere, despite the lesion involving mainly the dorsal portion of the left primary visual cortex. Importantly, we documented an activation of area hMT in the damaged hemisphere and the integrity of the entire visual pathways except for the optic radiations in the area of the lesion. In the third session we used an electrophysiological approach to study the neural basis of attention in the blind field of hemianopics. In order to obtain a reliable response with stimuli presented to the blind field, we used the Steady-State Evoked-Potentials (SSVEP) technique that is likely to be more informative than transient Visual Evoked Potentials in these kind of patients. This session included a passive stimulation and an attentional task. The former was performed to assess the response to stimuli flickering at a specific frequency in four visual field quadrants, two in the left and two in the right hemifield. In this session, we found in all hemianopic patients that visual stimuli presented to the blind hemifield produced a modulation of the neural response involving the damaged as well as the intact hemisphere. In the attentional task we found that orienting attention toward the blind field yielded an enhanced evoked response with respect to the non-orienting condition, even toward the blind field despite lack of perceptual awareness. Thus, SSVEP confirmed to be a useful means to assess a neural response following stimulus presentation in a blind field. In a broader perspective these results represent novel interesting evidence on the neural bases of unconscious vision in that they show that despite being unseen visual stimuli presented to the blind field elicit reliable neural activity in various cortical areas.

The temporal structure of neuronal spike trains in the visual cortex can provide detailed information about the stimulus and about the neuronal implementation of visual processing. Spike trains recorded from the macaque motion area MT in previous studies (Newsome et al., 1989a; Britten et al., 1992; Zohary et al., 1994) are analyzed here in the context of the dynamic random dot stimulus which was used to evoke them. If the stimulus is incoherent, the spike trains can be highly modulated and precisely locked in time to the stimulus. In contrast, the coherent motion stimulus creates little or no temporal modulation and allows us to study patterns in the spike train that may be intrinsic to the cortical circuitry in area MT. Long gaps in the spike train evoked by the preferred direction motion stimulus are found, and they appear to be symmetrical to bursts in the response to the anti-preferred direction of motion. A novel cross-correlation technique is used to establish that the gaps are correlated between pairs of neurons. Temporal modulation is also found in psychophysical experiments using a modified stimulus. A model is made that can account for the temporal modulation in terms of the computational theory of biological image motion processing. A frequency domain analysis of the stimulus reveals that it contains a repeated power spectrum that may account for psychophysical and electrophysiological observations.

Some neurons tend to fire bursts of action potentials while others avoid burst firing. Using numerical and analytical models of spike trains as Poisson processes with the addition of refractory periods and bursting, we are able to account for peaks in the power spectrum near 40 Hz without assuming the existence of an underlying oscillatory signal. A preliminary examination of the local field potential reveals that stimulus-locked oscillation appears briefly at the beginning of the trial.

Spatial navigation is a cognitive skill fundamental to successful interaction with our environment. Normal aging is associated with weaknesses in this skill, with severe deficits in the context of Alzheimer's disease. Identifying mechanisms underlying how the aged brain navigates is important to understanding these age-related weaknesses and potentially strengthening or preserving spatial navigation ability in the aging population. One understudied aspect of spatial navigation is self-motion perception. Important to self-motion perception is optic flow, which is the pattern of visual motion experienced while moving through our environment. Several brain regions are optic flow-sensitive (OF-sensitive), responding more strongly to optic flow than other types of visual motion. The goal of the experiments in this dissertation was to examine the role of visual motion perception and cortical motion area dynamics in spatial navigation in cognitively intact aged adults. Visual path integration tasks were used because they highlight the use of radial and translational optic flow to keep track of one’s position and orientation, respectively. In the first experiment, a positive relationship between radial optic flow sensitivity and visual path integration accuracy that was stronger in aged adults was found. In the second experiment, brain activity was measured using functional magnetic resonance imaging (fMRI) while participants performed visual path integration (VPI) and turn counting (TC) tasks. Stronger activity in the OF-sensitive regions LMT+ and RpVIP during VPI, not TC, was associated with greater VPI accuracy in aged adults. In the third experiment, the functional connectivity between OF-sensitive regions and the rest of the brain during the VPI and TC tasks was measured using fMRI. Stronger average functional connectivity between the OF-sensitive regions LMT+, RMT+, LpVIP, RpVIP, LpV6 and right supramarginal gyrus and posterior cingulate during VPI, not TC, was associated with greater VPI task accuracy in aged adults. The results demonstrate novel relationships between visual path integration accuracy and radial motion perception, the response of OF-sensitive cortical regions during visual navigation, and the interaction strength between OF-sensitive regions and parietal cortex during visual navigation in aged adults. This work expands our knowledge of mechanisms underlying spatial navigation processes in the aged human brain.

Les neurones du cortex visuel primaire (aire 17) du chat adulte répondent de manière sélective à différentes propriétés d’une image comme l’orientation, le contraste ou la fréquence spatiale. Cette sélectivité se manifeste par une réponse sous forme de potentiels d’action dans les neurones visuels lors de la présentation d’une barre lumineuse de forme allongée dans les champs récepteurs de ces neurones. La fréquence spatiale (FS) se mesure en cycles par degré (cyc./deg.) et se définit par la quantité de barres lumineuses claires et sombres présentées à une distance précise des yeux. Par ailleurs, jusqu’à récemment, l’organisation corticale chez l’adulte était considérée immuable suite à la période critique post-natale. Or, lors de l'imposition d'un stimulus non préféré, nous avons observé un phénomène d'entrainement sous forme d'un déplacement de la courbe de sélectivité à la suite de l'imposition d'une FS non-préférée différente de la fréquence spatiale optimale du neurone. Une deuxième adaptation à la même FS non-préférée induit une réponse neuronale différente par rapport à la première imposition. Ce phénomène de "gain cortical" avait déjà été observé dans le cortex visuel primaire pour ce qui est de la sélectivité à l'orientation des barres lumineuses, mais non pour la fréquence spatiale. Une telle plasticité à court terme pourrait être le corrélat neuronal d'une modulation de la pondération relative du poids des afférences synaptiques.
Primary visual cortex neurons in adult cat are selective to different image properties as orientation, contrast and spatial frequency. This selectivity is characterized by action potentials as electrical activity from the visual neurons. This response occurs during the presentation of a luminous bar in the receptive fields of the neurons. Spatial frequency is the amount of luminous bars in a grating presented from a precise distance from the eyes and is measured in cycles per degree. Furthermore, it was establish until recently that cortical organisation in the adult remains inflexible following the critical period after birth. However, our results have revealed that spatial frequency selectivity is able to change after an imposition of a non-preferred spatial frequency, also called adapter. Following cortical activity recordings, there is a shift of the spatial frequency tuning curves in the direction of the adapter. A second adaptation at the same non-preferred spatial frequency produced a different neural response from the first adaptation. This “short-term plasticity” was already observed in the primary visual cortex for orientation selective neurons but not yet for spatial frequency. The results presented in this study suggest that such plasticity is possible and that visual neurons regulate their electrical responses through modulation of the weights of their synaptic afferences.