Attentional effects on sensory tuning for single-feature detection and double-feature conjunction.

When humans scan their visual environment, relevant objects are selectively attended for enhanced processing. It is still unclear in what ways processing is modified by attention, and whether attentional selection operates on an individual feature (such as colour, orientation or motion) or on binding together different features. In the experiments reported in this paper, these two stages were characterized using psychophysical reverse correlation. Subjects viewed eight patches, briefly flashed and symmetrically arranged around fixation. Each patch consisted of segments that could vary in both colour and orientation. One of the patches ('target') differed from the remaining 'distractor' patches with respect to either its orientation, colour, or both (in three different experiments). Subjects were asked to detect the target patch. The stimulus was preceded by a cue. On some trials ('cued' trials), the cue informed observers that the target patch could only appear at two of the eight possible locations. On remaining ('uncued') trials, all eight positions were valid. Psychophysical reverse correlation was then applied to derive linear estimates of sensory filters for orientation only, colour only, and their conjunction. In line with the properties of single neurons in cortex, attentional cueing did not affect sensory tuning for detecting individual features. However, it affected the way in which features were subsequently (and very inefficiently) combined in a multiplicative fashion. The results are consistent with a model in which attention recalibrates internal responses to the statistics of the stimulus by having signals from different features mutually control each other through reciprocal inhibition.