Nasotemporal Overlap of Hemifields at Three Levels in the Visual Pathway: In primary visual cortex the decussation of retinal ganglion cells passes through the edge of the fovea (black circle), rather than through its centre, thus giving a region about 1 deg. wide that is represented in both hemispheres. Nasotemporal overlap increases at subsequent levels in the visual hierarchy because of callosal connections, until it is around 5 deg. in V4 and virtually the whole of the binocular visual field in inferotemporal cortex (IT). Note that. although the edge of the decussation is sharp, giving rise to a homonymous hemianopia if one hemisphere is lesioned, there is sufficient overlap between each edge of the decussation that foveal stimuli are projected to both hemispheres. In the experiments on binocular rivalry described here, the rivalling targets were small and foveal and thus activated both hemispheres, even in V1.
Fig. 2. Effect of Stimulus Strength on Binocular Rivalry
Increasing stimulus strength, (by increasing contrast, spatial frequency, velocity), leads to an increase in rivalry rate. This effect is least in individuals with the slowest rivalry rate (example f), who have a relatively shallow slope on the curve relating stimulus strength to rivalry rate. At very high rivalry rates, perception changes from rivalrous alternations to a continuous mixed percept (such as grid or crosshatch in this case where the rivalrous stimuli were horizontal and vertical gratings). Fast switchers (e..g.example a) show the steepest increase in rate as a function of strength. Although there is individual variation in the transition from rivalry to mixed percept, there is a broad ceiling at around 4 Hz that suggests that there is a fusional frequency limit for rivalry in this range.
Gradient of Switch Rates in the Putative Interhemispheric Switches:
Summary of the sketchy knowledge available about brainstem bistable oscillators and their cortical targets. Note that there is a rough gradient of switch rate in the brainstem, from caudal midbrain to hypothalamus, that matches a similar caudal-to-frontal gradient of switch rate in the neocortex. Because this information is still fragmentary (for example, some reviewers did not agree that there is a high speed oscillator driving V1 that is responsible for the extraordinary speed of the attentional spotlight of serial search, so there may be even more resistance to my postulating the dorsal tegmentum as the site of the oscillator in this diagram). Neverthless, this information grows in the direction that I have indicated, with recent confirmation of my prediction that even the hypothalamic suprachiasmatic nucleus (SCN) can act as a switch. While this piece was being written, new evidence emerged that the SCN of the hypothalamus can act as a bistable oscillator to bring about interhemispheric switching in the time scale of days (de la Iglesia et al 2000)
Global Account of the Oscillators that might be Involved in Rivalry:
The role of this diagram is to try to account for the individual variation in switch rate, as well as that fact that some individuals do not rival at all, seeing instead a mixed percept at all times. Since all individual tested can be induced to "abandon" rivalry by emotional releasers such as getting them to laugh, perhaps the individuals who never experience rivalry can be related to this temporary loss of rivalry in normals. Decreased stimulus strength can also abolish rivalry, as well as considerably slowing the rate before rivalry breaks down, so I have suggested a mechanism that could account for this effect. One could imagine that a weak stimulus has higher uncertainty and might therefore be more dependent upon higher processes like memory and imagination than a stronger stimulus that might drive posterior visual cortical processes directly. If the frontal regions switch more slowly (as the evidence suggests) one would therefore expect a low strength stimulus to be associated with slower switching. Why very low strength stimuli should also fail to produce rivalry is more mysterious, although this failure might be regarded as some kind of extrapolation to the infinitely slow rivalry rate expected at the lowest stimulus strength . Work in progress in my laboratory (Hekel, in preparation) suggests that the apparent losses of rivalry at both high and low stimulus strengths are unrelated phenomena that share a subjective similarity that does not stand up to closer examination.. The failure of H/V rivalry at high stimulus strengths appears to reflect the upper temporal limit of rivalrous alternations that can be perceived. This interpretation is supported by Keith Whiteís experiments with eye swapping and rivalry where aliasing or beat frequency phenomena are elicited systematically as the rate of dichoptic alternation is increased (White et al 2001). High speed dichoptic alternations are clearly having an effect on the system, but rivalry is incapable of following at high speeds that exceed the limit around a few hertz. I have already pointed out the surprising slowness of this limit when it is compared with other temporal limits in the visual system, which are an order of magnitude faster.
In contrast, the apparent failure at low stimulus strengths may be a different phenomenon that seems to share features with patchwork rivalry, where the patches are so numerous and the stimulus so uncertain that the subject reports the presence of the orthogonal orientations simultaneously over the whole display, even though the two orientations cannot actually be perceived to cross over at any location. For this reason it may superficially resemble the grid perception that occurs above the temporal limit of rivalry at high stimulus strengths. Note that in this latter case there are visible intersections between horizontal and vertical contours at all locations in the display, a percept that our limited number of subjects are so far not willing to admit in the low contrast case. The task of reporting rivalry becomes very difficult at low stimulus strengths we think, not only because rivalry may slow down and cease, but because the stimulus becomes so weak that it is difficult to rule out what we think is a likely interpretation: viz: that patchwork rivalry is now occurring at every resolvable location in the display. Given the very different appearance of the H/V rivalry display at high- versus low- stimulus strength, we do not think it appropriate to view both apparent losses of rivalry as related phenomena, even if the explanation I have just provided needs modification as we study more subjects, bearing in mind once again the considerable variation in individual perception of precisely the same rivalry display.
At least the diagram maps out the rivalry phenomena that have
to be accounted for and emphasises the difficulties of so doing if one
confines oneís attention to the properties of V1.