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

Abstract

Visual latencies, and their variation with stimulus attributes, can provide information about the level in the visual system at which di¡erent 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 di¡erent stimulus attributes were processed by mechanisms at the same or di¡erent 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 £ux, 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, di¡er reliably, and all are longer than the reaction time to a change in light £ux. On average, observers take 184 ms to detect a change in light £ux, 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 di¡erent 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 di¡erent 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.

Goal-driven modulation of oculomotor capture

“…That is, it may be that there is a restricted time window within which onsets can delay movement initiation. The similarity effect is presumably dependent upon color processing in the ventral extrastriate areas, which is likely to be slower than the processing of a luminance onset (Barbur, Wolf, & Lennie, 1998;Schmolesky et al, 1998). Therefore, the color-based similarity effect may take longer to develop (Ludwig & Gilchrist, 2003), and exert a more sustained influence on the response latencies.…”

A linear chromatic mechanism drives the pupillary response

“…The latency of the pupillary response to the test stimuli was calculated using the normalization technique developed by Barbur et al (1998). The time-courses for the earlier phase of the response (i.e.…”

“…jL7Mj+jL + Mj). Some researchers (Young & Alpern 1980;Barbur et al 1998) have demonstrated a di¡erence in the latency of pupillary responses between isoluminant and luminance stimuli, thereby suggesting that the balance between the di¡erent mechanisms is dependent on visual stimuli.…”

“…The constriction and dilation of the pupils is evoked by changes in the ambient luminance. However, in addition to changes in retinal illuminance, the response of the pupils to visual stimuli can also be determined by factors such as cognition ) and changes in stimulus colour (Alpern & Campbell 1962;Kohn & Clynes 1969;Young & Alpern 1980;Krastel et al 1985;Barbur et al 1992Barbur et al , 1998Young et al 1993;Kimura & Young 1995, 1996Gamlin et al 1998).…”

The effects of task and saliency on latencies for colour and motion processing

“…As described in more detail below, our low-saliency condition was defined as the 99% detection level for a colour or motion change. The high-saliency level was the point at which RTs became almost asymptotic (Barbur et al 1998).…”