Dynamics of early visual cortical activations

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Early spread of visually evoked responses

We conducted and analyzed data from large number of visual experiments, and in most cases the earliest visually evoked cortical responses showed similar activity pattern. These are examples from  two different experiments where checkerboard pattern and face stimuli were presented to subjects.

Early visual activity spread

Early spread of visual responses. Spread of activity from the initial response in V1 as a function of time. Typical examples taken from two different experiments (upper and lower rows) for the stimuli presented in the lower quadrants of visual field. The inset in the upper left corner of each row indicates the stimuli for each experiment. In the upper row axial MRI slices are used, in the lower row the MRI slices that best cover the dorsal part of V1 are shown. Yellow contours encompass the regions of statistically significant (P < 0.005) activations. Activations in V1 that were clearly separated are labelled accordingly. Latencies of activations are given on each MRI.

The spread of the stimulus-evoked activity within the visual system is very rapid, with activity reaching many extrastriate areas by the time the V1 peak around 70 ms is reached.

V. Poghosyan, A. A. Ioannides, Neuron 58, 802-813 (2008). PDF >>    Suppl PDF >>

V. Poghosyan, A. A. Ioannides, Neuroimage 35, 759-770 (2007). PDF >>

Onset latencies of early visual cortical responses

Three male subjects passively viewed checkerboard pattern stimuli presented at eight different locations in the parafoveal (4� eccentricity) and peripheral (9�) visual fields. For each subject the same experiment was repeated on three different days, months apart.

Peripheral vs. parafoveal visual field stimulations

Onset latencies of early visual areas. Mean and SD of the onset latencies of early visual areas for each subject, separated for peripheral and parafoveal stimulations. Each row represents latencies of an area indicated on the left side of the figure. The circle, asterisk, and square denote different subjects as specified in the upper right corner of the figure. Blue and red colours represent latencies for peripheral and parafoveal stimulations, respectively. The symbol for each subject is placed at the position of the mean onset latency, and the surrounding bars show the SD across all runs (3 runs for each of 3 days). POS, parieto-occipital sulcus; vPOS, ventral POS; dPOS, dorsal POS.

Here we show the onset latencies of early visual area activations and their dependence on the stimulated location of the visual field. The results showed for the first time significantly shorter onset latencies in V1 for peripheral than parafoveal visual field stimulations. POS, parieto-occipital sulcus; vPOS, ventral POS; dPOS, dorsal POS. 

V. Poghosyan, A. A. Ioannides, Neuroimage 35, 759-770 (2007). PDF >>

Interactions between areas V1 / V2 and human MT+

We examined the interactions between V1/V2 and hMT+ at different stimulus contrasts using moving and stationary stimuli presented centrally or peripherally.

V1 hMT+ interactions

Information flow between V1/V2 and hMT+. Summary of information flow between V1/V2 and hMT+ at high (A) and low (B) contrasts. Latencies for estimated shift in averaged response are given by the horizontal position of the boldface abbreviations. Latencies for the estimated shift in response variances are indicated by the horizontal bars, and show a similar trend to the shift in averaged response. Bar length reflects the duration over which Mutual Information value was significant. The abbreviations indicate the latencies of: Lateralization effect (L); Vertical position effect (V); Motion effect (peripheral, MP; central, MC). Pairs of horizontal bars connected by thin arrows indicate the latencies for periods of significant relatedness in response variance between V1/V2 and hMT1, as revealed by Mutual Information Analysis. For a description of relatedness indicated by colour, see the paper (Figures 8 and 9).

At high contrast, responses to stimulus motion and position began in V1/V2, and then moved to hMT+ with a 34 - 55 ms delay. Whereas, at low contrast, the direction of the information flow was reversed: lateralized responses in hMT+ came first, with those in V1/V2 lagging with a delay of 27 ms. The results suggest that feedback from hMT+ for low-contrast stimuli compensates for unresolved processing in V1/V2 when the input of a visual image is weak.

M. Maruyama et al., Human Brain Mapping 30, 147-162 (2009). PDF >>