The contribution of LM to the neuroscience of movement vision

Abstract

The significance of early and sporadic reports in the 19th century of impairments of motion vision following brain damage was largely unrecognized. In the absence of satisfactory post-mortem evidence, impairments were interpreted as the consequence of a more general disturbance resulting from brain damage, the location and extent of which was unknown. Moreover, evidence that movement constituted a special visual perception and may be selectively spared was similarly dismissed. Such skepticism derived from a reluctance to acknowledge that the neural substrates of visual perception may not be confined to primary visual cortex. This view did not persist. First, it was realized that visual movement perception does not depend simply on the analysis of spatial displacements and temporal intervals, but represents a specific visual movement sensation. Second persuasive evidence for functional specialization in extrastriate cortex, and notably the discovery of cortical area V5/MT, suggested a separate region specialized for motion processing. Shortly thereafter the remarkable case of patient LM was published, providing compelling evidence for a selective and specific loss of movement vision. The case is reviewed here, along with an assessment of its contribution to visual neuroscience.

Keywords: akinetopsia; cerebral motion blindness; movement vision; patient LM.

Figure 1

Oculomotor scanning patterns during the inspection of a scene (A) in an age-matched normal subject (B) and in LM (C). Dots indicate fixation positions, lines saccadic eye shifts. Both subjects reported all relevant items. Scanning time was 13.6 s for the normal subject and 26.6 s for LM. Note similar correspondence of scanning patterns to the spatial configuration of the scene in both subjects.

Figure 2

Reading eye movement patterns in an age-matched normal subject (A; same as in Figure 1B) and in LM (B). Dots indicate fixation positions. Reading performance in the normal subject was 156 words per minute (wpm), in LM 72 words per minute. The slowness in LM can be explained by a higher number of fixation repetitions (22.7% in LM vs. 4.3% in the normal subject) and in longer fixation durations (0.31 s on average in LM vs. 0.22 s in the normal subject).

Figure 3

Proportion of “no” (gray bars), “uncertain” (hatched bars), and “yes” responses (dark bars) of LM in 20 trials in stimulus velocities ranging from 2°/s to 20°/s. Moving path length was 20°. LM's task was to indicate verbally, whether she can see the stimulus in motion (yes responses), was not sure about motion (uncertain responses) or could not see motion at all (no responses; 10 trials per velocity). Presentation time was unlimited, but was usually between 2 and 5 s. Note increase in “no” and decrease in “yes” responses with increasing velocities.

Figure 4

Mean subjective velocities for LM in 1982 (circles) and in 1990 (diamonds), and in an age-matched normal subject (squares; same as in Figures 1B, 2A; 10 trials per velocity) as a function of stimulus velocity calculated from motion prediction responses (modified after Zihl et al., 1983, 1991). LM was instructed to press a key to start the target in motion, and to press it again when she judged that the now invisible target had reached a red marker behind a mask. The path of horizontal movement was 10°; the length of the path behind a mask was 20°. Note that motion prediction accuracy in LM dropped for stimulus velocities >6°/s on both occasions.

Figure 5

Comparison of LM's performance in a motion-coherence task with that of MT-lesioned monkey (Newsome and Paré, 1988). (A) Threshold coherence values for the Movshon noise stimulus as a function of spatial stimulus displacement for LM (filled circles) and an age-matched normal subject (open circles; same as in Figure 4). (B) Same as (A), but for monkeys before (open diamonds) and after acute MT lesion (filled diamonds). Presentation time was 1 s. The subjects were required to indicate the perceived (or guessed) direction of stimulus motion (left or right). Note the similarity in the effect of brain injury to motion coherence perception (modified after Baker et al., 1991).

Figure 6

(A) Recordings of LM's smooth pursuit eye movements to a target moving either at 4°/s (upper trace) or at 8°/s (lower trace). Note deterioration of smooth pursuit at the higher velocity (modified after Zihl et al., ; © Oxford University Press with permission). (B) Handwriting with eyes open (upper writing) and eyes closed (lower writing). Time taken for writing was 4 s with eyes closed and 26 s with eyes open (modified after Heywood and Zihl, , © Psychology Press with permission). Note better writing with eyes closed.