This experiment looks at how adaptation to a spatial pattern influences our perception of another spatial pattern. The experiment is similar to that of Blakemore and Sutton (1969) but it uses a method of constant stimuli instead of a method of adjustment. The aim of this study was to measure the effects of different adaptation durations and 87 participants took part in one of three different conditions to test this. The hypothesis was that the PSE (point of subject equality) would be higher after more time of adaptation. A computer program was used to present the stimuli to the participants and the results were then recorded. The ANOVA revealed that the PSE was significantly higher after 20 seconds adaptation compared to no adaptation. However, the duration of the adaptation period had little effect on the results; 5 seconds of adaptation produced a similar PSE mean compared to 20 seconds of adaptation.
Introduction
Adaptation is the process whereby the brain adapts its response to a constant stimulus over time. In this experiment, we are attempting to recreate the work of Blakemore and Sutton (1969) which evaluated how adaptation to a spatial pattern influences our perception of another spatial pattern. They suggested that adapting to one spatial frequency changes our perception of others. The spatial frequency is the frequency with which a periodic pattern changes over time. The methodology has been improved for this experiment and instead of using a method of adjustment, a method of constant stimuli was used.
The experiment will systematically measure the effect of adapting to one spatial frequency on our perception of other frequencies. Systematically measuring the Motion Aftereffects, will enable us to understand how the brain processes motion. By studying the effects of adaptation, psychologists can find out exactly how the brain processes information and in this case, spatial information. There will be three different conditions involved in obtaining the results. The participants will either take part in experiment 1, 2 or 3 which have been designed to calculate the PSE based on the participants responses.
If you stare at a pattern for a long period, the visual system becomes adapted. The processes underlying adaptation are not fully understood. Adaptation may be passive (neurones become fatigued) or active (recalibration). However, what is clear is that adaptation to a high contrast pattern has the result of making it more difficult to see a low contrast pattern. The question arises whether adaptation to a single spatial frequency affects the whole of the Contrast Sensitivity Function or just sensitivity to the test frequency.
Blakemore and Campbell (1969) measured the Contrast Sensitivity Function before and after adaptation to a sine wave of a particular spatial frequency. They found that contrast thresholds were elevated only for a limited range of spatial frequencies close to the adapting frequency. They concluded that adaptation had isolated a particular channel in the brain and the CSF was the envelope of a number of overlapping spatial frequency selective channels. In 1968, Campbell and Robson suggested that the CSF does not reflect the sensitivity of a single mechanism, but the combined activity of many independent mechanisms (called ‘filters’, ‘detectors’, or ‘channels’)
If the visual system splits the image up into separate spatial frequency bands then interactions between channels may have effects on perception. Blakemore and Sutton (1969) found that, after adaptation to a sine wave grating of a particular spatial frequency a grating of a lower spatial frequency appeared to be lower still, and grating of a higher spatial frequency appeared to be of an even higher spatial frequency.
An explanation of the spatial frequency shift requires a model of how activation in a range of channels gives rise to the perception of a specific spatial frequency. One possibility is that spatial frequency is encoded in terms of the distribution of activity across a population of spatial frequency coded channels. Blakemore and Sutton suggested that adaptation to a lower spatial frequency reduces the response in that channel to the test spatial frequency and so shifts the peak of the distribution away from the adapting spatial frequency. Another possibility suggested by DeValois and DeValois is that spatial frequency is encoded in terms of the relative activation of tuned channels.
If there are two (A, B) adjacent overlapping spatial frequency tuned channels then if B is more active than A indicates a high spatial frequency and vice versa. If the test grating is chosen to activate A and B equally, then adaptation to a high spatial frequency will reduce the sensitivity of B so that, when the test is replaced, A is now greater than B, which indicates a shift to a lower spatial frequency. These two types of explanation are common in many areas of visual science. One problem with the population code approach is that any pattern will give rise to a distribution of activity across channels. If perception is determined by the most active channel, it is difficult to explain why we see patterns other than gratings.
A similar psychophysical approach can be used to show that we have orientation specific channels. Contrast adaptation is orientation specific as well spatial frequency specific (Blakemore and Campbell, 1969). In addition, an orientation specific aftereffect can be measured. The tilt after effect, which is similar to the spatial frequency shift. Adaptation to a particular orientation has the effect of shifting the apparent orientation of adjacent orientations away from the adapting orientation. These results indicate that channels are tuned to both orientation and spatial frequency.
The fact that adaptation to contrast is orientation tuned, suggests that the site of adaptation is at or beyond striate cortex, which is the first location in the retino-cortical pathway in which orientation tuned cells can be found. DeValois, Albrecht and Thorell (1982) measured the orientation and spatial frequency tuning of V1 cortical cells in the Macaque monkey. They found neurones only responded to a narrow range of orientations and spatial frequencies.
Thorell, used 2-deoxyglucose as an indicator of neural activation and found that, when a monkey viewed low spatial frequency gratings, activity was greater near cytochrome oxidase blobs, but when high spatial frequencies were viewed, activity was greatest in the interblob regions. This indicates that spatial frequency channels are ordered from blob regions to inter-blob regions. Orientation channels are organised so that an orderly progression in orientation preference around the blobs can be found.