Adaptation to a high temporal frequency visual stimulus has been found to produce a distorting effect on perceived duration which is specific to the area of visual field occupied by the adapting stimulus (Johnston et al., 2006). Specifically, adaptation to a drifting grating which is spatially localised and has alternating direction of motion has an effect on subsequently presented stimuli in that area of the visual field and nowhere else: when the test stimulus has a temporal frequency of 10Hz, adaptation to a 20Hz grating produces a temporal compression. This effect was found using both forced choice and reproduction paradigms (see Figure 36).
These visual adaptation stimuli do not influence the perceived duration of tones. The results of this study suggest the involvement of spatially localised temporal mechanisms in the perception of brief visual stimuli. They also have implications with regard to the locus of timing mechanisms involved in sub- second time perception: This is because if a moving grating were to affect the pacemaker of a central timing mechanism then all spatial positions would be affected and the position of the adaptor stimulus would be irrelevant. This would seem to undermine the model of a central, dedicated timer for humans, at least for durations under 1 second.
Although perceived temporal frequency was affected by both high (reduced) and low (Increased) frequency adaptation, the duration distortion effect was found to be specific to high temporal frequency stimuli only. In other words the duration compression found was independent of perceived temporal
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frequency. Adaptation to a stimulus of 5 Hz produced little effect on perceived duration.
Figure 36Spatially specific duration adaptation. A) Subjects adapted to a oscillating grating. Subsequently they were required to judge the relative durations of two moving gratings, one of which was in the same position as the adaptor stimulus and one of which was in a different position in visual space. B) Denotes the perceived duration of a 600ms grating after adaptation to gratings drifting at 5Hz and 20Hz. Test gratings could be at the same orientation as the adapting stimulus or at 90 degrees. C) Perceived onsets and offsets of post adaptation gratings relative to a burst of white noise. Perceived duration of post adaptation grating measured by reproduction. A= adaptation, S= standard, C= comparison, T= test, R= response (Johnston et al., 2006).
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The authors suggest that post adaptation duration compression may arise via a shift in temporal impulse responses in the magnocellular pathway (Johnston et al., 2006).
A related finding has also been reported by Burr et al. The authors state that these spatially specific adaptation effects occur in real world coordinates. In other words they occur in a particular area of external space which is independent of head or eye position and not in a particular area of the retina (retinotopically) (Burr et al., 2007b). The authors postulate that the locus for this compression of perceived duration lies at a very late stage of the visual system. This hypothesis is based on the spatiotopic nature of the adaptation effect found (external space is represented retinotopically throughout nearly all of visual system) and the results of another experiment which found that adaptation to a drifting grating in one eye produced a spatiotopic effect in the other.
It should be pointed out however, that a later study found no such effect (Bruno et al., 2010). A spatiotopic adaptation effect would imply that the locus of the timing mechanism involved is at a higher level than that mooted by Johnston et al (2006) since external space is not thought to be represented spatiotopically until some areas of the parietal cortex (Galletti et al., 1995, Duhamel et al., 1997). Burr et al (2007) suggest the lateral intraparietal area as a possible candidate. This area has receptive fields which are affected by saccades, after which a remapping of the external world is necessary in order
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to maintain continuity of visual perception (Duhamel et al., 1992). Saccades have also been shown to distort duration perception (Burr et al., 2007b). However, spatiotopic duration adaption to a drifting grating is not a universal finding. A number of experiments have claimed that this phenomenon is in fact, retinotopic (Bruno et al., 2010).
In addition, spatial adaptation has been found with adaptors as narrow as 1 degree of visual angle (Ayhan et al., 2009b). The fact that adaptation has been found for such a narrow visual stimulus points to an early part of the visual pathway being responsible for this effect since the receptive fields of cells in the visual cortex beyond V1 are too large to show sensitivity to changes in the position of such small adapting stimuli. Wherever this effect is located, the idea of a central pacemaker would seem to be incompatible with these findings as it would be expected that adapting to a stimulus in a particular area of the visual field should produce an effect equally across retinotopic or spatiotopic space.
The magnocellular-LGN pathway has been suggested as a likely to locus for perceived duration compression following adaptation to a high temporal frequency stimulus (Johnston et al., 2006, Ayhan et al., 2009b, Bruno et al., 2010). This hypothesis is supported by the observation that magnocellular neurons are particularly responsive to high temporal frequency stimuli since significant duration compression was not found when adapting stimuli were lower in temporal frequency (5Hz).
The model which may be used to explain the perceived duration compression outlined above is known as a “content dependent clock model”. This model
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suggests that time is measured by predicting how a visual stimulus will appear after a fixed period of time so that when the stimulus matches the prediction the aforementioned period has lapsed and the clock is reset. This model links timing to the visual perception of stimuli with changing characteristics. Perceived duration compression is said to be produced due to differential adaptation effects in magnocellular and parvocellular neurons. To date there does not appear to be a hypothesis which explains how spatiotopic duration compression following adaptation to a drifting grating could occur in the absence of a retinotopic component. To sum up, that a spatially specific compression following adaptation to a high temporal frequency visual stimulus is well established. However, whether this effect is spatiotopic or retinotopic has yet to be proven one way or the other.
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