General Summary and Conclusion

This project reports a systematic spatial sensitivity shift that follows the focus of attention - on a trial-by-trial basis and at identical eccentricities and under identical task difficulty - with various approaches: The RF shift was evident in the comparison of conditions with attention to different subregions inside the RF (inside-inside comparison, sect. 3.3.1) and with attention to subregions inside versus attention outside the RF (inside-outside comparison, sect. 3.3.2). Furthermore the position change of the RFs was evident in the center of mass and peak position of one dimensional projections of the RF activity profile (slice analysis), as well as in the peak position of the two dimensional RF maps. Additional analysis of the shift showed that there was no bias of the shift towards the fovea (vector analysis, sect.

3.3.2). This analysis also showed that RFs shifted towards the stimulus inside the condition with no bias towards the stimulus outside the RF, showing that RFs shift to the inside stimulus when attended and towards the outside stimulus when that stimulus is attended, which replicates the attention shift reported by Connor et al.

(1997).

Moreover, in contrast to an asymmetric attentional RF shift towards only one

’fixed positon’ of the RF, which would be suggestive of a static ’attentional RF center’ of single neurons (Connor et al., 1997), we showed that asymmetries in the extent and direction of RF shifts are due to stimulus arrangements and that RFs show a strong directional specificity: RFs shifted more towards an attended stimulus the further away they were from that stimulus in theattend outside condition (sect.

3.3.3). Further direct implication for underlying mechanisms can be derived from our temporal analysis of the response to the RF probe stimulus, which showed that

3Attention has also been reported to be generally more limited in spatial resolution than ’retinal’

vision (Intriligator and Cavanagh, 2001). It should also be noted, that some authors (Intriligator and Cavanagh, 2001) suggest that attention is generally limited in spatial resolution. This claim is based on the suggestion that the difference of ’individuation’ of a particular item (e.g. are there two or three lines in the visual field) and the detection of the presence of items (e.g. are there lines present in the visual field) is followed by a difference of attentive versus non-attentive processes.

We do not follow this reasoning and thus do not try to link it with our main neurophysiological finding.

sustained peripheral attention inducessustained shifts of RFs, in contrast to possible transient effects due to the abrupt luminance onset of the probe stimulus (sect. 3.5).

Our finding of a small marginally significant shrinkage of RFs with attention inside the RF (sect. 3.3.4) complicates the general picture somewhat with regard to the possible underlying mechanisms of attention involving inter-neuronal compe-tition, ormultiplicative gain modulation of afferent neuronal populations. However, the moderate shrinkage of RFs was not related to the systematic shift of RFs (sect.

3.3.4), nor was it related to modulation of neuronal response strength (sect. 3.3.5).

In summary, the project was able to reveal that selective attention gradually routes information flow by partial shifts of receptive fields towards the attentional focus. The reported finding is grounded on well characterized and controlled be-havioral conditions and neuronal responses: First, neurons were confirmed to be located in area MT based on histological reconstruction (sect. 2.1.2) and function-ally inferred from strong and consistent tuning to the direction of motion with two independent methods (sect. 3.1.1). Secondly, the behavioral performance, i.e. the focus of attention, was constant across conditions and ensured an equal task diffi-culty level (sect. 3.2.1). Thirdly, directional specificity of the RF probe responses were not confounded by eye position shifts (sect. 3.6). Fourth, we reproduced the basic and classical spatial attention effect of stronger neuronal responses with atten-tion inside compared to outside the RF - when we considered the response to the stimuli inside the RF in the absence of the RF probe stimulus (sect. 3.2.2). This last aspect is particularly noteworthy because we obtained the RF shift in response to the probe stimulus in the absence of additional attentional modulation of response strength to the probe.

These findings lead to several general conclusions: First, RFs shift towards the focus of attention without equally strong systematic RF size changes. Such a gradual shift with enhanced activity close to the focus of attention and reduced sensitivity to probes far from it is consistent with a gain hypothesis of attention that assumes attentional modulation to act on the responses of afferent, presynaptic neuronal pop-ulation. Accordingly, area MT neurons would integrate already modulated responses from neurons with smaller receptive fields that closely match the spatial extent of the attended stimuli (such as in primary visual cortex) either by a feedforward, or a recurrent mechanism. While the functional consequence of the shift is qualitatively consistent with the effect expected from the RF shrinkage hypothesis, it does not reflect an exclusive gating mechanism with an emphasis on inhibitory mechanisms that a shrinkage around attended stimuli implies and which is often insinuated as spotlight, or beam metaphors of selective spatial attention. More clearly, we do not observe a shrinkage around the attended stimulus.

The reported shift of single neuronal spatial sensitivity in extrastriate area MT

reflects at the population level the additional recruitment of neuronal resources at the focus of attention. This mechanism could be the neural correlate of various per-ceptual effects that are centered on the focus of attention, including (i) distributions of enhanced processing accuracy close to the attentional focus and suppression in its surround, (ii) enhanced spatial resolution, and (iii) distortions in spatial judgements.

Overall, this project revealed for the first time a high degree of spatially specific plasticity of neuronal RFs due to selective visual attention with implications for models about the mechanisms of attention and about the relation of perceptual phenomena and modulations at the level of single cortical neurons.