Abstract
Purpose:
Directly comparing the motion and form processing in neurologic disorders has remained difficult due to the limitations in the experimental stimulus. In the current study, motion and form processing in amblyopia was characterized using random dot stimuli in different noise levels to parse out the effect of local and global processing on motion and form perception.
Methods:
A total of 17 amblyopes (8 anisometropic and 9 strabismic), and 12 visually normal subjects monocularly estimated the global direction of motion and global orientation in random dot kinematograms (RDK) and Glass patterns (Glass), whose directions/orientations were drawn from normal distributions with a range of means and variances that served as external noise. Direction/orientation discrimination thresholds were measured without noise first then variance threshold was measured at the multiples of the direction/orientation threshold. The direction/orientation and variance thresholds were modelled to estimate internal noise and sampling efficiency parameters.
Results:
Overall, the thresholds for Glass were higher than RDK for all subjects. The thresholds for both Glass and RDK were higher in the strabismic eyes compared with the fellow and normal eyes. On the other hand, the thresholds for anisometropic amblyopic eyes were similar to the normal eyes. The worse performance of strabismic amblyopes was best explained by relatively low sampling efficiency compared with other groups (P < 0.05).
Conclusions:
A deficit in global motion and form perception was only evident in strabismic amblyopia. Contrary to the dorsal stream deficiency hypothesis assumed in other developmental disorders, deficits were present in both motion (dorsal) and form (ventral) processing.
Amblyopia is a neurodevelopmental disorder characterized by reduction in either monocular or binocular vision without any organic cause. The primary site of the visual deficit in amblyopia is reported to be the striate cortex.
1–3 However, the functional visual deficits in amblyopia seem to spread beyond primary visual cortex to extra striate cortical areas.
4–8
Global motion direction judgment or perceiving a global orientation from the distribution of locally oriented elements is accomplished in two stages where the local direction and orientation are initially encoded in early visual areas of primary visual cortex/area V2 (V1/V2), after which the global processing is thought to occur at higher extra striate cortical areas; middle temporal area (MT), medial superior temporal (MST) for motion, and V4 for form.
9,10 Behavioral studies in humans have reported reduced sensitivity to both global motion and global form in amblyopia.
11–15 The motion coherence threshold remained elevated even after the local deficits from V1/V2 such as in contrast sensitivity was accounted for.
13,14,16,17 However, other studies have reported normal performance in amblyopes using a different experimental design where the global motion direction judgement was evaluated in varying degrees of local directional noise.
15,18 Global form is also compromised in amblyopes as observed by abnormal global form coherence thresholds for Glass patterns (Glass) and translational/rotational line segments.
12,19,20 However, other studies have reported inconsistent results where discrimination of the mean global orientation from an array of Gabor patches with different orientations is reported to be normal,
15 slightly reduced,
11 or even abnormal.
21
Several reviews of global motion and form perception in a range of developmental disorders
22–24 including amblyopia
12 indicate that the motion processing is more affected than the form processing. According to the ‘two-streams hypothesis',
25 the global motion and form perceptions are processed along the dorsal and ventral streams, respectively, bifurcated from early cortical areas.
26–28 There have been a number of attempts to compare these two domains of vision by measuring the outputs from the processing of the corresponding stream using similar stimuli. For example, studies used translational random dot kinematograms (RDK) for motion and oriented streaks for form created by superimposing the multiple still frames from the respective RDK to compare the amblyopic performance in two streams.
12,29 The results showed deficits along both visual processing streams, with a relatively larger deficit in motion processing (dorsal stream) compared with the form processing (ventral stream). The superimposed orientation streaks are considered, however, no different from line segments where the global integration of local elements does not seem necessary to do the task.
22 It is also difficult to ascertain if the deficit observed in either or both streams is purely stream-specific, mere collateral damage from the early areas (V1/V2) common to both streams, or a combined effect of both. Direct comparison of the two streams can be made more precise by making stimulus parameters as similar as possible except for the domain specific outputs of interest.
Many psychophysical studies of global processing mechanisms have used coherence tasks where one measures the minimum proportion of coherent elements to random elements needed for a reliable discrimination. However, this paradigm is limited in separating the local influence of the early visual areas (V1/V2) shared by both streams from the global aspect of visual processing by the higher extra striate areas specific to each stream. This limitation can be resolved by measuring outputs in varying levels of noise added to the stimuli.
30
In the current study, we measured global motion and form discrimination thresholds in normal and amblyopic subjects to probe and compare the differences (or similarities) in their motion and form processing using similar stimulus with and without external noise to investigate how their performance are affected by the local and global parameters of the task.