Attention modulated responses to one and two stimuli within the receptive field from a typical cell. Responses are organized according to attended position and stimulus conditions within the receptive field. Spike trains were convolved with a Gaussian of 75 ms standard deviation and only responses occurring prior to any change within the receptive field were included. The neuron displayed strong orientation tuning: at both positions within the receptive field (Position 1: right column, Position 2: top row). The responses to single Gabors was proportionally increased when attention was directed within the receptive field across both positions and the three orientations tested (solid vs dashed). The attentional modulation of responses to pairs of Gabors within the receptive field was more complex: for the same visual stimulus, responses often differ depending on the particular position within the receptive field that was attended (thick vs thin).

Attentional modulation as a function of attended position and presented stimuli for the average neuron. The responses of each neuron were sorted according to the ranking of single stimulus responses with respect to orientation and position and normalized according to the response rate seen in the Attend Out condition with a single stimulus of best orientation at the optimal position. The format is identical to that of the previous figure: single stimuli responses include the average responses to preferred, intermediate, and null and well as the average spontaneous activity, while paired stimuli responses include the average responses to pair-wise combinations of those orientations (9 points) as well as spontaneous activity. Unlike the example cell of Figure 3, neither position dominates paired responses in the Attend Out conditions (A, left). However, similar to the previous figure, paired stimulus responses are dominated by the stimulus at the attended position when attention is directed within the receptive field (A, center and right). Attentional modulation is well fit by linear models for both single stimulus (B and C) and paired stimulus (D and E) responses.

Effects of attention for all cells with significant orientation tuning. Mean attentional modulation is not significantly different for single stimuli of preferred and null orientation (A and B). For both cases, the median (triangle) is significantly larger than one. Paired responses in which a Gabor of preferred orientation is paired with a Gabor of null orientation are shown in C–E. When the preferred stimulus is attended, large attention modulations were observed with a median of 1.64. When the null stimulus is attended, responses are also increased but significantly less than the increase seen when preferred stimuli were attended: the median attention modulation is 1.03. A direct comparison of responses when attention is shifted from preferred to null stimulus within the receptive field is shown in E.

Performance of attention models in which paired stimulus responses with different attentional loci (Attend In Pos 1, Attend In Pos 2, and Attend Out) are predicted on the basis of Attend Out single stimulus responses. Crosses represent fits of the average neuron shown in Figure 4. As with Figure 7, model error is defined by the sum of weighted residuals divided by the explainable variance. Filled circles indicate individual neurons whose fit was significantly different (F-test, p<0.05) between the two compared models. The filter model, in which attentional modulation occurs at the unattended position, is worse (p<0.001, paired sign rank) at explaining responses than the spotlight model, in which attentional modulation occurs at the attended position (A): most points are above the diagonal line. A generalized model, the input gain model (Equation 5), in which attentional modulation is free to occur at both the attended and unattended positions, outperforms both the filter (B) and spotlight (C) models. Triangles indicate median error values. The poorest fit to the data was provided by the output gain model (D).

Monkeys performed a peripheral orientation change detection task, in which they release a lever immediately after a change occurred at cued position while ignored changes at all other positions. Stimuli were counter-phasing Gabors placed at 4 positions: 2 within the receptive field and 2 in the diagonally opposite quadrant. Cues were instruction trials prior to the beginning of block of trains in which the position of a single stimulus indicated the behaviorally relevant position for the subsequent block. Within a block, trials in which one Gabor was presented within the receptive field (A) were interleaved with trials in which two Gabors were presented (B). For example, in a block in which the cued position was position 1, the monkey would be required to ignore the orientation change at position 3 (center), and only release the lever when the orientation at position 1 changes (right).

Attentional modulation as a function of attended position and presented stimuli for the data of Fig 2. Stimuli are organized in the same format: preferred orientation at position 1 is on the left while preferred orientation at position 2 is along the top. Responses are color-coded along a plane that represents orientation at position 1 (x-axis) and orientation at position 2 (y-axis). Responses to pairs of Gabors are shown in the square regions: elongated bars illustrate the responses to single Gabors. For this neuron, the stimulus at position 1 dominates the paired responses when attention is directed outside of the receptive field (A, square left), despite the fact that the single stimulus responses at the two positions are very similar (A, bars left). When attention in directed within the receptive field, the stimulus at the attended position (bars) dominates the response to paired stimulation (A, center and right square). Responses to identical visual stimulation but varying attentional locus are plotted for single stimuli (B and C), and paired stimuli responses (D and E). Single stimuli responses includes those to three different orientations (preferred, intermediate, and null) and well as spontaneous activity assessed during pre-stimulus epochs is included. Paired stimuli responses include those the combinations of the three orientations (9 points) as well as spontaneous activity. Error bars indicate +/− 1 SD. Attentional modulation for single stimuli is well fit by a linear model in which attention to position 1 (B) and position 2 (C) multiplicatively increases responses. For paired responses, a linear model also provides a good fit when attention is directed to position 1 (D). However, when attention is directed to position 2 (E), the linear model fails because of the poor correlation (r=0.529) between responses when attention was directed to position 2 (A right square) and when attention was directed outside of the receptive field (A left square). Confidence intervals for the model parameters are at p<0.05.

Average effects of attention over all responses and neurons grouped according to the number of stimuli within the receptive field. On average, both single stimulus (A, left) and paired stimulus responses (B, right) are well fit by a linear model which includes a multiplicative term as well as a offset. The multiplicative (gain) term is significantly different from unity (p<0.05) and the offset term, although small, is significantly larger than zero (p<0.05). The models are virtually identical for the two classes of stimuli, indicating that the fundamental mechanisms of attention are unlikely to be dependent on the number of stimuli within the receptive field. Mean attentional modulation, as defined by the ratio of the responses between Attend In and Attend Out conditions, is indistinguishable for the two classes of stimuli (B and D). Triangles indicate the median attentional modulation.

Performance of spatial summation models in which paired stimulus responses are predicted on the basis of single stimulus responses. Model error is the MSE of the spatial summation model weighted by the variance of each observation and divided by the explainable variance. Triangles indicate median error values. Three models are extremely poor at fitting the experimental observations: winner-take-all (A), averaging (B) and unscaled power (C). Two single-parameter models, scaled linear (D) and normalization (E), in which a scaling term is applied to the sum of single stimulus responses, have a median error of around 0.60. The best model, in which both the scaling and power term of the equation 1 are free to vary, has the best median performance, and provides better fits (F-test, p<0.05) than the scaled linear (F) and normalization (G) models for many individual neurons (filled circles) and overall (paired sign rank).

Input gain attention parameters for neurons whose model error was less than 0.5 plotted on a log-log scale (N=35). Each neuron contributes two points: one set of parameters when attention is directed to position 1 and another when attention is directed to position 2. Parameters then sorted according to attended and unattended positions (A). Points beneath the diagonal (gray) have more facilitation at the attended position than at the unattended position. Diagonal line indicates points consistent with the output gain model; dashed line, points consistent with the spotlight model, and dotted line, points consistent with the filter model. Attentional modulation was significantly larger at the attended position (p<0.001, paired sign rank) over the population. Triangles indicated median values, p values indicate significance of difference from a gain of 1.0 (t-test). Gain is significantly greater than one at the attended position, and not significantly less than one at the unattended position. Attentional gain parameters are plotted as a function of the spatial summation model used in B (scaled linear) and C (normalization). Parameters are close to the diagonal in both cases for most cells and are not significantly different overall, indicating that they are not strongly influenced by the particular model of spatial summation used.