Neeme Kahusk

The Effects of Coarseness of Quantisation, Exposure Duration, and Selective Spatial Attention on the Perception of Spatially Quantised (‘Blocked’) Visual Images

The two objectives of the present article are (a) to present a brief overview of the effects of systematic variation of the spatial-scale value of quantisation and stimulus duration on the identification of original images that have been degraded by Harmon–Julesz type of image pixelisation by ‘blocking’ and (b) to report the results of two experiments where the effects of selective spatial precuing by local and global peripheral precues on the identification of quantised target stimuli have been studied. Both the overview and the new results reported here demonstrate some counterintuitive effects: (1) abrupt decrease in identification efficiency with only a minor change in the coarseness of quantisation over a critical value of pixels per stimulus; (2) a cost for valid attentional precuing with coarse-quantised images. If physical precues (exposed with stimulus onset asynchrony of 120 ms) were employed in order to orient spatial attention to perceive original or fine-quantised stimuli, then attentional facilitation was found. However, if the precued stimuli were coarse quantised then the facilitative effect crossed over to a detrimental effect of attention. These effects are discussed in the context of the microgenetic approach that presupposes the existence of a perceptual–attentional processing routine that operates according to the coarse-to-fine time-course rule of selective attentional activation of stimulus representations at various spatial scales.

Time-course of Spatial-attentional Focusing in the Case of High Processing Demand on the Peripheral Precue

Subjects had to identify pairs of spatially overlapping or neighbouring letters exposed successively at various locations around the central fixation. The first letter (S1) in a pair acted both as a physical precue to indicate the S2 location and as a stimulus letter to be identified. The second letter (S2) in a pair acted only as a letter to be identified. The SOA was varied between 0 and 320 ms in different experiments. Comparison of the time-course functions of S2 identification in the conditions of (1) single pair presentation and (2) presentation of the letter pairs in the context of distractor letters that accompanied S2 in time, but occupied alternative spatial positions, revealed that S2 processing from the covert spatial-attentional focus was open to non-decreasing interference from the distractor letters up to 240 ms SOAs. An unexpected result consisted in equal level of S2 identification regardless of whether S1 had to be identified or not. This shows that if the peripheral precue has to be pro...