REEMPLAZO DE LA BOMBILLA

This thesis examined the neural pathways that underpin our ability to perceive MID, focusing on two binocular cues – CD and IOVD. Previous research, reviewed in Chapter 1, has revealed cortical sites that can be driven in a feed-forward manner by both cue types. The work presented here adds to this body of research by probing these neural mechanisms in more detail, asking how information necessary for the computation of CD and IOVD is represented in a hierarchy of visual areas. Key questions included the neural locus of CD processing, how information in early precortical pathways contribute to MID processing, and how eye-specific information necessary for computing IOVD is maintained in extrastriate visual areas. Finally, attentional feedback mechanisms were isolated to investigate how activity in areas associated with CD and IOVD processing is affected by task demands.

In the first experimental chapter, it was demonstrated that signals carried in all three precortical pathways can contribute to MID mechanisms, in a manner analogous to 2D motion mechanisms. Contrast-scaled achromatic, isoluminant L-M, and S-cone isolating stimuli were able to drive MID percepts in stimuli that isolated the CD and IOVD mechanisms. Adding noise to the stimulus degraded the ability of participants to discriminate the direction of the MID, and the IOVD signal was more susceptible to noise than the CD stimulus. However, masking the chromatic signal with achromatic noise elements did not affect performance. These findings provided the first demonstration that both CD and IOVD mechanisms draw on signals carried in chromatic pathways.

The second experimental chapter described fMRI responses to CD and IOVD stimuli, where responses were driven in a feed-forward manner. The effects of disparity and 2D motion, as well as other low-level properties, were removed by subtracting responses to control stimuli. First, this study replicated previous findings that implicated hMT+ in CD and IOVD computation. Responses to both stimulus types were measured throughout a hierarchy of visual areas, with no overall differences between the two cue types. Secondly, this study found no evidence that the putative CSM area is particularly critical for computing CD.

Although responses to CD stimuli were correlated with those measured in hMST, they were generally weaker and as such contributed only to a lesser extent to the CD mechanism.

This study also discriminated between achromatic and S-cone inputs to CD and IOVD mechanisms. Although no overall differences in chromaticity were measured, there was a significant interaction between MID cue type and chromaticity. S-cone IOVD cues elicited particularly strong responses, even in primary visual cortex, where responses were larger even than to the achromatic IOVD stimulus. This suggests an early, binocular motion- opponent signal that is most effectively carried in the S-cone pathway.

In contrast, the S-cone contribution to the CD signal was weak. This is somewhat surprising given that S-cone and achromatic stimuli were found to contribute equally to CD MID perception in the first experimental chapter. Taken together, these two findings would imply that whilst the S-cone pathway can contribute to disparity mechanisms, and whilst S- cone CD MID can be perceived, these signals do not translate to large BOLD modulations in motion-sensitive areas.

However, it is more likely that the experimental paradigm used in the psychophysical study was not sensitive enough to detect such differences in chromatic pathway input. Indeed, the measured thresholds were generally high and the slopes of the psychometric functions were shallow, indicating a high degree of uncertainty. It is suggested that the dissociation between S-cone inputs to psychophysical judgements of CD or IOVD-defined MID might be more carefully teased out by simplifying the experimental task (for example, to a two-alternative forced-choice task where participants choose the interval that contains MID). This may be more suited than the challenging task of identifying the initial direction of MID in a stimulus that moves in both directions within a single trial. I would predict that, under this type of paradigm, the results would show a strong koniocellular contribution to the IOVD mechanism, and a weak contribution to the CD mechanism, and thus would dovetail with the fMRI findings presented in this thesis.

The third experimental chapter investigated whether eye-specific signals remain available in cortical areas that are involved in the computation of IOVD, both inside and outside primary visual cortex. To increase the sensitivity of the analysis to detect information encoded on a fine scale, multivariate pattern analysis and classification techniques were

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applied to fMRI data. Nonetheless, the analysis was limited by the difficulty in estimating reliable event amplitudes in a dense event-related design. EOO information was decoded in V1, and motion direction was detected in hMT, but no regions where both sources converged were identified. This suggests that IOVD mechanisms inherits EOO information from ocular dominance columns in V1. Increasing the stability of generating beta estimates may reveal the presence of this information in extrastriate areas.

Finally, the fourth experimental chapter investigated how top-down, attentional demands modulate activity in those areas that can be driven by bottom-up CD and IOVD signals. Detecting changes in CD and IOVD-defined MID resulted in net changes in BOLD amplitude, with strong attentional effects measured as early as V1. Task demands therefore influenced neural activity even at very early stages. These changes were not driven by bottom-up changes in the stimuli, and varied instead depending on which aspect of the stimulus the participant was attending to. Furthermore, responses in hMT and hMST were correlated with the correct detection of CD and IOVD targets, demonstrating that neural populations in these areas are critically involved in detecting CD and IOVD. This study provided the first demonstration that neural populations in these motion-sensitive areas are crucially involved in detecting changes in MID, and are recruited by attentional mechanisms.

The work presented here supports the prevailing view that both CD and IOVD share common cortical loci, although they draw on different sources of visual information, and that signals for both cue types are multiplexed in areas known to be involved in the computation of binocular disparity and 2D motion. The overarching task of computing MID draws on many different sources of information. These include estimates of direction, speed and depth. Because motion signals and depth signals are inherently linked in the natural world, the combination of multiple sources of information computed though parallel pathways is likely to improve the quality of MID estimates extracted across a dynamic range of features.