Visual processing levels revealed by response latencies to changes in different visual attributes.

Visual latencies, and their variation with stimulus attributes, can provide information about the level in the visual system at which different attributes of the image are analysed, and decisions about them made. A change in the colour, structure or movement of a visual stimulus brings about a highly reproducible transient constriction of the pupil that probably depends on visual cortical mechanisms. We measured this transient response to changes in several attributes of visual stimuli, and also measured manual reaction times to the same stimulus changes. Through analysis of latencies, we hoped to establish whether changes in different stimulus attributes were processed by mechanisms at the same or different levels in the visual pathway. Pupil responses to a change in spatial structure or colour are almost identical, but both are ca. 40 ms slower than those to a change in light flux, which are thought to depend largely on subcortical pathways. Manual reaction times to a change in spatial structure or colour, or to the onset of coherent movement, differ reliably, and all are longer than the reaction time to a change in light flux. On average, observers take 184 ms to detect a change in light flux, 6 ms more to detect the onset of a grating, 30 ms more to detect a change in colour, and 37 ms more to detect the onset of coherent motion. The pattern of latency variation for pupil responses and reaction times suggests that the mechanisms that trigger the responses lie at different levels in cortex. Given our present knowledge of visual cortical organization, the long reaction time to the change in motion is surprising. The range of reaction times across different stimuli is consistent with decisions about the onset of a grating being made in V1 and decisions about the change in colour or change in motion being made in V4.