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Miguel Eckstein, Dorion Liston, Richard Krauzlis; Non-equivalence between attentional modulation and increases in signal contrast for superior colliculus neurons. Journal of Vision 2007;7(9):342. doi: 10.1167/7.9.342.
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Visual attention in detection tasks is typically manipulated by the presence of a cue that predicts the position of the signal with some validity (i.e., 75 %). A large number of studies have shown how attention modulates neuronal activity in a variety of areas including V1, V2, V4, and FEF (Reynolds & Chelazzi, 2004). A current concept is that attention affects the neuronal response in a similar manner to changes in signal contrast. This statement has been mostly based on measurements of average spike rate but most typically without a Receiver Operating Characteristic (ROC) analysis assessing how a neuron's ability to discriminate between target and distracter is affected. Here, we analyze activity of saccade-related neurons in the superior colliculus (SC) in a cueing task with three signal contrast levels to determine whether attention alters neurons' abilities to discriminate between signal and distracter in a similar way to changes in signal contrast. We recorded from eighty five saccade-related neurons in the SC of two monkeys during a luminance discrimination task (2 alternative forced choice) with a 75% valid cue. Saccade performance (percent correct) increased with signal contrast and was higher for the trials in which the signal appeared at the cued location (valid cue trials) vs. the uncued location (invalid cue trials). Session to session cueing effect amplitudes predicted using ROC analysis of SC neuronal activity correlated with the measured behavioral cueing effects. Consistent with previous studies, SC neuronal activity (average spike rate) increased with both attention and signal contrast. However, ROC analysis indicated that the ability of the individual neurons to discriminate between signal and distracter varied with signal contrast but did not with attention (cue presence). We conclude that for the present task and for SC neurons attentional modulation is not equivalent to variations in signal contrast.
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