Seyed-Reza Afraz

Retinotopy of the face aftereffect.

Physiological results for the size of face-specific units in inferotemporal cortex (IT) support an extraordinarily large range of possible sizes--from 2.5 degrees to 30 degrees or more. We use a behavioral test of face-specific aftereffects to measure the face analysis regions and find a coarse retinotopy consistent with receptive fields of intermediate size (10 degrees -12 degrees at 3 degrees eccentricity). In the first experiment, observers were adapted to a single face at 3 degrees from fixation. A test (a morph of the face and its anti-face) was then presented at different locations around fixation and subjects classified it as face or anti-face. The face aftereffect (FAE) was not constant at all test locations--it dropped to half its maximum value for tests 5 degrees from the adapting location. Simultaneous adaptation to both a face and its anti-face, placed at opposite locations across fixation, produced two separate regions of opposite aftereffects. However, with four stimuli, faces alternating with anti-faces equally spaced around fixation, the FAE was greatly reduced at all locations, implying a fairly coarse localization of the aftereffect. In the second experiment, observers adapted to a face and its anti-face presented either simultaneously or in alternation. Results showed that the simultaneous presentation of a face and its anti-face leads to stronger FAEs than sequential presentation, suggesting that face processing has a dynamic nature and its region of analysis is sharpened when there is more than one face in the scene. In the final experiment, a face and two anti-face flankers with different spatial offsets were presented during adaptation and the FAE was measured at the face location. Results showed that FAE at the face location was inhibited more as the distance of anti-face flankers to the face stimulus was reduced. This confirms the spatial extent of face analysis regions in a test with a fixed number of stimuli where only distance varied.

Adaptation to apparent motion in crowding condition

Visual adaptation has been successfully used for studying the neural activity of different cortical areas in response to visual stimuli when observers do not have explicit conscious access to those stimuli. We compared the orientation selective adaptation to apparent motion and its effect on the perception of stimuli with bistable apparent motion in crowded and non-crowded conditions. In the crowding paradigm conscious access to a visual stimulus is severely impaired when it is flanked by other similar stimuli in the peripheral visual field. As expected, adaptation to the target stimulus occurred in the non-crowded condition in all of the individual subjects (n=4; P<0.001). Although in the crowded condition subjects were not able to discriminate the target stimulus, adaptation to that stimulus was still preserved (P<0.001). There was no significant difference between the adaptations in the two conditions of the apparent motion (P>0.05). Imaging studies have shown that V5 cortex is the earliest visual area that specifically responds to apparent motion. Our results suggest that in certain conditions V5 may be activated while there is no explicit conscious access to the apparent motion.

Orientation-selective adaptation to crowded illusory lines.

Visual adaptation has been successfully used as a psychophysical tool for studying the functional organisation of visual awareness. It has been shown that orientation-selective adaptation to a grating pattern occurs in crowded conditions. In such conditions, simultaneous presentation of flanking distractors pushes the target stimulus out of conscious perception and severely impairs orientation discrimination in the periphery of the visual field. In the present study, orientation-selective adaptation to illusory lines induced by two line gratings abutting each other with a phase shift was examined in crowded and non-crowded conditions. To rule out the effects of lower level adaptations we used an animation paradigm in which the orientations of the two line gratings were altered repeatedly during adaptation phase without any change in the orientation of the resulting illusory line. Although performance of subjects in reporting the orientation of crowded illusory lines was at chance level, orientation-selective adaptation was preserved for crowded as well as non-crowded adapting targets. Two control experiments demonstrated that adaptation to endpoints of real lines at the location of abutting grating lines had minimal effect on the adaptation to illusory lines; and changes in the configuration of endpoints could not be responsible for better performance when adapting and test stimuli were different. We conclude that a crowding effect occurs after illusory lines have been processed in the visual stream. Since illusory lines seem to be represented at relatively early stages of visual processing (eg area V2), adaptation to crowded illusory stimuli suggests that neuronal activation in those early stages is not necessarily correlated with conscious perception.

Erratum: Microstimulation of inferotemporal cortex influences face categorization (Nature (2006) 442 (692-695))

This corrects the article DOI: 10.1038/nature04982

Motion-induced overestimation of the number of items in a display

Subjects were asked to report the number of items in a display as the items moved along a circular path around the fixation point. As the rotation speed increased, the apparent number of items also increased. This motion-induced overestimation (MIO) effect was investigated in three experiments. In the first experiment, the effect of rotation speed and set size was explored with an enumeration task. The overestimation error increased with an increase in speed or number of items in the display. In the second experiment, we used an adjustment paradigm to measure the speed threshold of MIO effect onset. Temporal rate of the display, which was defined as product of rotation speed and the number of rotating items, was the determining factor of MIO onset. In the third experiment, moving items were marked with different colours. Surprisingly, the number of perceived items was still overestimated even though the number of perceived colours was not.

Visual spatial integrity in the absence of splenium.

Several lines of evidence have confirmed that visual integrity among the two hemifields is severely impaired following transection of posterior part of the corpus callosum (splenium). But the extent of this impairment differs for various visual functions. Here, in a posterior callosectomized patient (MD), interhemispheric visual spatial integrity is investigated in three experiments. In the first experiment, MD was asked to make decision about horizontal alignment of two balls presented simultaneously in one of three conditions: both in right or left hemifield, or each in one hemifield. We have previously shown that she is not able to perform a shape-matching task for stimuli presented to different hemifields. But in this task, MD was able to compare location of the two bilaterally presented stimuli significantly above chance level. Then we investigated whether attentive visual object tracking across vertical meridian of the visual field is possible in the absence of splenium. MD had to attentively track one bouncing ball among three identical balls, while it crossed the vertical midline in half of trials. Her performance in crossed conditions was significantly above chance level, but it was lower than uncrossed conditions. Finally, we investigated the contribution of simple interhemispheric temporal signals in performing the attentive tracking task. Results suggest that the patient was not using such temporal signals. Our results suggest that interhemispheric connections other than splenium can contribute in making an integral visual map across hemifields. Such an integrated map can be used for bilateral visual spatial comparisons and visual spatial attention.

Tactile Discrimination Task Not Disturbed by Thalamic Stimulation

The major pathway of human somatosensation passes through the ventrocaudal nucleus (Vc) of the thalamus. We tested the effect of direct electrical stimulation of the Vc on tactile discrimination in 5 Parkinson patients undergoing stereotactic thalamotomy. Raised gratings with lines 3, 4, or 6 mm apart were used. Patients had to actively touch test patterns placed in the hand contralateral to the thalamus under operation and compare it with a reference 3-mm grating held continuously in their other hand. Their performance was best for 6-mm, followed by 3-mm and then by 4-mm patterns regardless of electrical stimulation. Under Vc stimulation, patients recognized the 4-mm gratings slightly better. This can be explained either by the nature of thalamocortical interaction, which makes it resistant to external noise, or by the involvement of other pathways that circumvent the effect of thalamic stimulation.