Spatial Attention Effects

The main hypothesis of this project concerns a change of the spatial position and size of RFs when attention is directed to stimuli at different subregions of the RF and when attention is brought from a region outside the RF to a region inside the RF. The analysis of attentional effects on spatial sensitivity sets this project apart from most previous attentional studies which were primarily concerned with changes in response amplitude. It is therefore relevant to provide at the outset of the spatial analysis some further background results to validate our approach and to ease comparison with other studies.

3.2.1 Behavioral Performance

As a prerequisite for all later analysis, our experimental settings succesfully ensured comparable performance levels in the attentional conditions. Figure 3.5 illustrates that average hit rates did not differ systematically between the attentional condi-tions. The monkey performed on average 79.3% correct in the attention tasks. Apart from errors involving early or later lever releases and hits we recorded fixation er-rors in 16.8% of all the attention trials. While an ANOVA with condition as factor (including the sensory control conditions) revealed a main effect of condition (F = 33.90, p<0.01), Tukeys comparison tests showed that attention conditions (attend inside to S1 or S2 andattend outside to S3) did not differ to each other (all p<0.05) but each of them differed to the easier fixation with S1S2 task (all p<0.01)).

3.2.2 Modulation of Response Strength

Directing spatial attention inside the RF of a neuron typically enhances response (cf. p. 16). However, few studies have investigated the effect of spatial attention inside the RF compared to outside the RF when there are two stimuli in the RF that both move in the non-preferred direction of the neuron (e.g. Treue and Martinez-Trujillo 1999; Martinez-Martinez-Trujillo and Treue 2004). We computed the attentional modulation index (cf. section 2.3, p. 58) between both conditions based on the average firing rate of the neuron in response to the pair of stimuli (S1 and S2) moving in the non-preferred direction in intervals during the trial when there wasno probe stimulus presented (i.e. during the sequence of probe presentation but when there was no probe shown). Figure 3.6, A, shows the distribution of attentional modulation indices for the set of n=57 cells. Positive index values reflect enhanced firing rates in theattend inside compared to theattend outside condition. We found an average AI of 0.078, or 16.7% after conversion from index values to percentage, which is a statistically significant enhancement of responses due to spatial attention

(one sample t-test, two-tailed, p<0.05).

In order to test for the influence of the RF probe stimulus on the strength of attentional modulation we selected the RF probe at the spatial position at which it elicited the maximum average response after subtraction from the response in the same condition but in the absence of the probes (i.e. when only S1 and S2 was present). We then computed the attentional modulation index based on this probe response in the attend-outside (i.e. attend S3) condition and in the attend-inside conditions (attend S1 or S2). We did not find a significant attentional modulation of response strength for this comparison (cf. figure 3.6, B). The difference of this comparison to the previous analysis (fig. 3.6,A) is only the presence of the RF probe in the latter analysis. To summarize these results, we find an enhanced response strength with attention inside the RF in the absence of the RF probe. Subtracting thisbaseline modulation from the response to the compound of S1, S2and RF probe does not result in any further modulation with spatial attention inside compared to

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fixation errors attend outside (S3) attend inside (S2) attend inside (S1) fixation with S1,2,3

Figure 3.5: Performance rates of the monkey in the attentional tasks (attend S1, S2 and attend outside to S3) and the neutral fixation task with identical stimulation (fixation with S1S2 and S3). Early (late) responses signify error trials in which the monkey responded before (after) the direction change of the target stimulus happened or did not respond at all (included in late responses). Performance levels involving the lever (hits, early and late responses) were computed from all trials during the experimental condition after subtracting the number of fixation errors.

Fixation errors included trials when the monkeys gaze left the predefined circular window around the fixation square and are presented as proportion to all trials.

The presented average values are based on the performance during 61 experiments during which cells were recorded that went into the RF analysis.

A B

Figure 3.6: Modulation of response strength with attention outside versus inside the RF. A: Attentional influence on responses to the stimuli inside the RF moving in the non-preferred direction in the absence of the probe. We find on average 16.7%

enhanced responses with spatial attention inside compared to outside the RF. B: Attentional influence on the response to three stimuli inside the RF (S1, S2 and the probe stimulus moving in the preferred direction of the cell). There was a non-significant trend for a lower response to the three stimuli in the attend-inside compared to the attend-outside condition. Histograms depict the distribution of attentional indices with the geometric mean provided in the figures reflecting the converted mean index value. Arrows in the graphs show the mean value of the distribution with the small flanking bars representing the pm0.95 confidence limit of the mean.

outside the RF.

To summarize, we find different influences of attention in the presence and ab-sence of the probe stimulus in the RF: Without probe stimulus, spatial attention to S1 or S2 enhanced the responses to S1/S2 by 15.7% relative to the response to S1/S2 when S3 was attended. With probe stimulus present, the response to the probe - after subtraction of the response to S1/S2 - was not further modulated by attention.

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Figure 3.7: Average PSTH to the RF probe stimuli for each of 57 cells which resulted in the maximum average firing rate in the attention conditions (dark grey bars). Light grey bars represent the PSTH during the temporal interval of the attention conditions (i.e. in the presence of S1 and S2), but in the ab-sence of the RF probe stimulus moving in the preferred direction. Response in the latter condition served as a baseline for the RF analysis.