Spatial Attention Decorrelates Intrinsic Activity Fluctuations in Macaque Area V4

Abstract

Summary Attention typically amplifies neuronal responses evoked by task-relevant stimuli, while attenuating responses to task-irrelevant distracters. In this context, visual distracters constitute an external source of noise that is diminished to improve attended signal quality. Activity that is internal to the cortex itself, stimulus independent ongoing correlated fluctuations in firing, might also act as task-irrelevant noise. To examine this, we recorded from area V4 of macaques performing an attention-demanding task. The firing of neurons to identically repeated stimuli was highly variable. Much of this variability originates from ongoing low frequency (<5 Hz) fluctuations in rate correlated across the neuronal population. When attention is directed to a stimulus inside a neuron’s receptive field these correlated fluctuations in rate were reduced. This attention-dependent reduction of ongoing cortical activity improves the signal to noise ratio of pooled neural signals substantially more than attention-dependent increases in firing rate.

Optogenetically induced low-frequency correlations impair perception

“…We find that low-frequency optical stimulation 1 modulates the timing of the neural response (Fig 4D) but does not alter the overall 2 magnitude of the population response (Fig 4A,B,C). We replicate previous findings that 3 attention reduces low-frequency spike-count correlations in the baseline (no optical 4 stimulation) condition (Mitchell et al, 2009) (Fig 3A, left panel; gray versus white bars, 5 ! = 0.02, t-test).…”

“…Spike trains were tapered 11 with a single Slepian taper, giving an effective smoothing of 2.5Hz for the 400ms 12 window (NW=1, K=1). To control for differences in firing rate between the two 13 conditions, we adopted a rate matching procedure similar to (Mitchell et al, 2009). 14 Induction of coherent activity in the network due to sub-threshold sinusoidal stimulation 15 was calculated as a modulation index of coherence across the two conditions: 16 SSC MI = (SSC with -SSC without )/(SSC with +SSC without ).…”

Paying Attention to the Cortical Layers

“…Third, the influence of attention on the variability of neuronal responses, as measured with the Fano factor, were relatively modest and confined to broad-spiking cells in the superficial layers. Fourthly, Nandy et al (2017) replicated previous findings that attention reduces noise correlations (Cohen and Maunsell, 2009;Mitchell et al, 2009), but in the present study this reduction was largely confined to the input layers of cortex and did not occur in the deep layers. This finding is problematic for theories that emphasize that attention works by reducing noise correlations, because a reduction in the output layers of cortex would be essential if downstream areas are to benefit from this effect.…”

Stimulus-dependent variability and noise correlations in cortical MT neurons

“…Importantly, both preferred and nonpreferred stimuli reduced the FF. In addition, the evoked noise correlation-that is, the trial-to-trial covariance of stimulus induced activity between two simultaneously recorded neurons-is also reduced upon stimulus presentation (18), after stimulus adaptation (19) or perceptual learning (20), and under attention (14,21), an effect that could, under certain conditions, lead to more reliable estimates of the mean population activity (22). Hence, there is a growing body of evidence suggesting that the encoding of a signal through cortical activity may be improved by minimizing both trialby-trial variability and noise correlations.…”