The effect of simultaneous flickering light stimulation on global form and motion perception thresholds.

The question regarding the exact function of the primary visual cortex (V1) in vision has been around ever since the description of residual vision after damage to this cortical area by Riddoch in 1917. In 2002, Schoenfeld and colleagues proposed that V1 can be saturated by flashes of light, by which the function of V1-bypassing visual pathways can be "unmasked". The Schoenfeld group found that light flashes applied on stimulus onset led to the elevation of brightness increment detection thresholds, but left motion detection thresholds unaltered. Although the proposed method (i.e. the use of light flashes to induce refractoriness in V1) could be a simple, cheap and elegant way of exploring V1 functions, no study has followed up on this. Therefore it is not known if it works at all with other types of stimuli. For that reason, we decided to revisit the idea in a modified form. Global form and motion perception thresholds were assessed with static Glass pattern stimuli and random dot kinematograms, with and without 12Hz flickering light stimulation. Global motion thresholds were almost unaltered by flickering stimulation, while a significant threshold elevation was caused in the global form perception task. The strongest conclusion allowed by our data is that simultaneous flickering photostimulation elevates global form perception thresholds but not global motion perception thresholds. This is in some way related to the refractoriness generated in an unsatisfactorily defined part of V1. We suggest that this does not necessarily reflect the activity of V1-bypassing pathways, and propose that the application of light flashes is a method that deserves more attention in the exploration of the V1-dependent and independent elements of visual consciousness in human subjects.

Effects of embryonic light stimulation on the ability to discriminate left from right in the domestic chick.

Following an early suggestion by Mach [Mach E. The analysis of sensations. Chicago: Open Court Publishing House; 1897 [reprinted by Dover Publications, 1959]] it has been claimed that brain asymmetry would be crucial for biological organisms in order to discriminate left from right. However, direct evidence in support of this hypothesis is scanty. In the animal system model provided by the newly hatched domestic chick (Gallus gallus) it has been proved feasible to manipulate lateralization on visual tasks by exposing the eggs to light for a brief period before hatching. The light exposure leads to the development of lateralization of some visual functions and generates asymmetry in the thalamofugal visual projections to the forebrain, because the late-stage embryo is turned in the egg so that it occludes its left but not its right eye. Thus, it is possible to obtain organisms with strong (light-incubated chicks, Li-chicks) or weak (dark-incubated chicks, Di-chicks) lateralization. Li- and Di-chicks were trained to discriminate between two small beads for food reward on the basis of their relative left-right position. Li-chicks performed better than Di-chicks. In order to check whether Di-chicks showed a general impairment in discrimination learning, not confined to left-right discrimination only, chicks were tested in a spatial re-orientation task in a square-shaped arena, in which a target located in a corner could be identified using the left-right location of a conspicuous cue (the colour of a wall) which could be used as a landmark. Results showed that Di-chicks were impaired with respect to Li-chicks in use of left-right information for re-orientation, but not in use of the cue as a landmark. These results provide direct evidence that modulation of the strength of visual lateralization may affect left-right discrimination abilities.

Magnetic stimulation of the right visual cortex impairs form-specific priming.

Recent evidence suggests that priming of objects across different images (abstract priming) and priming of specific images of an object (form-specific priming) are mediated by dissociable neural processing subsystems that operate in parallel and are predominantly linked to left and right hemispheric processing, respectively [Marsolek, C. J. Dissociable neural subsystems underlie abstract and specific object recognition. Psychological Science, 10, 111-118, 1999]. Previous brain imaging studies have provided important information about the neuroanatomical regions that are involved in form-specific and abstract priming; however, these techniques did not fully establish the functional significance of priming-related changes in cortical brain activity. Here, we used repetitive transcranial magnetic stimulation (rTMS) in order to establish the functional role of the right occipital cortex in form-specific priming [Kroll, N. E. A., Yonelinas, A. P., Kishiyama, M. M., Baynes, K., Knight, R. T., & Gazzaniga, M. S. The neural substrates of visual implicit memory: Do the two hemispheres play different roles? Journal of Cognitive Neuroscience, 15, 833-842, 2003]. Compared to no TMS and sham TMS, rTMS of the right occipital cortex disrupted immediate form-specific priming in a semantic categorization task. Left occipital rTMS, on the other hand, had no converse effect on abstractive priming. Abstract priming may involve deeper semantic processing and may be unresponsive to magnetic stimulation of a single cortical locus. Our TMS results show that form-specific priming relies on a visual word-form system localized in the right occipital lobe, in line with the predictions from divided visual field behavioral studies [Marsolek, 1999].

Polarization contrast and motion detection.

Form and motion perception rely upon the visual system's capacity to segment the visual scene based upon local differences in luminance or wavelength. It is not clear if polarization contrast is a sufficient basis for motion detection. Here we show that crayfish optomotor responses elicited by the motion of images derived from spatiotemporal variations in e-vector angles are comparable to contrast-elicited responses. Response magnitude increases with the difference in e-vector angles in adjacent segments of the scene and with the degree of polarization but the response is relatively insensitive to the absolute values of e-vector angles that compose the stimulus. The results indicate that polarization contrast can support visual motion detection.

Effects of light stimulation of embryos on the use of position-specific and object-specific cues in binocular and monocular domestic chicks (Gallus gallus).

Chicks hatched from eggs incubated in the dark (D-chicks) or from eggs exposed to light during the last 3 days before hatching (L-chicks) were trained on day 4 to peck at small cones for food reinforcement. The cones had different patterns (checked or striped) and were located in different positions (either on the left or on the right of a rectangular arena) so as both object-specific (pattern) and position-specific cues could be used to discriminate cones that contained or that did not contain food. After learning, the position of the cones was reversed so that object- and position-specific cues provided contradictory information. No effect of light incubation was observed in binocular chicks that chose cones on the basis of object-specific cues. Monocular D-chicks also tended to approach and peck the cones with the correct pattern in the wrong position, whereas monocular L-chicks did not show any clear choice. Initial choices for one side or other of the arena were mostly determined by the first side visible through the non-occluded eye in D-chicks, particularly when using their left eye. These results suggest that light exposure of the embryo makes neural mechanisms that do not receive direct visual input (i.e., those of the occluded side) more available to be used in assessment of novelty.

The effect of expectation on facilitation of colour/form conjunction tasks by TMS over area V5.

In an earlier paper, we reported task-specific impairments and improvements caused by applying TMS over cortical visual area V5 [Proceedings of the Royal Society of London B 265 (1998) 537]. The phenomenon is further investigated in the present study using two of the previous tasks: a motion/form conjunction in which TMS impaired performance and a colour/form conjunction in which performance was enhanced with TMS. In the earlier experiment, subjects were presented with blocks of trials of one task type perhaps allowing some of the observed effects to arise from knowing the type of stimulus to be discriminated. When blocks of trials consisted of randomly mixed moving/form and colour/form conjunction tasks, TMS over V5 still impaired target-present responses for the moving/form conjunction, but the facilitation seen for colour/form conjunction target-present responses disappeared. We suggest that the competitive inhibition postulated between visual movement areas and colour areas in the brain, in our previous paper, are subject to expectation or knowledge of forthcoming stimulus type.