Cerebellar dysfunction of movement and perception.

This review describes some characteristics of patients with cerebellar lesions, including limb movements, changes in motor planning and disturbances in time-dependent perception. The delay in movement initiation can be explained by a delay in onset of movement-related discharge of neurons in motor cortex. Disorders of movement termination (hypermetria) are accompanied by asymmetric velocity profiles and by prolonged agonist and delayed antagonist EMG activity necessary to brake the movement.

Asymmetry in visual search for targets defined by differences in movement speed.

Perception of motion speed was investigated with the visual search paradigm, using human Ss. When searching for a fast target among slow distractors, reaction time was minimally affected as the number of distractors was increased. In contrast, reaction time to detect a slow target among fast distractors was slow and linearly related to the number of distractors.

Timing and motor control in clumsy children.

The study examined timing control in clumsy children and employed the Wing-Kristofferson (1973) model of repetitive movements in an attempt to identify the locus of timing control difficulties in clumsy children. Two groups of children classified as normal and clumsy (ages 6-7 and 9-10) performed tapping and perception of duration and loudness tasks. Results indicated that clumsy children were significantly more variable than normal children in maintaining a set rate of tapping and in accurately judging time intervals.

Is the cerebellum involved in learning and cognition?

Current studies are examining whether the cerebellum has a functional role in non-motor tasks using both behavioral and physiological methods with animals, and computer simulations of a classical conditioning task. Cerebellar involvement in cognition has been assessed in studies with healthy and neurologically impaired humans. The results have led to new hypotheses that are providing testable predictions about the role of the cerebellum in perception, attention, and other cognitive functions.

Density effects in conjunction search: evidence for a coarse location mechanism of feature integration.

Four experiments used the visual search paradigm to examine feature integration mechanisms. Reaction time to determine the presence or absence of a conjunctive target is relatively fast and exhaustive for low-density displays. Search rate is slow and self-terminating for high-density displays.

Timing and force control deficits in clumsy children.

This study investigated the link between cognitive processes and neural structures involved in motor control. Children identified as clumsy through clinical assessment procedures were tested on tasks involving movement timing, perceptual timing, and force control. The clumsy children were divided into two groups: those with soft neurological signs associated with cerebellar dysfunction and those with soft neurological signs associated with dysfunction of the basal ganglia.

Impaired velocity perception in patients with lesions of the cerebellum.

In three psychophysical experiments, cerebellar patients were impaired in making perceptual judgments of the velocity of moving stimuli. Performance was normal when the judgment concerned the position of the stimuli (Experiment 1). The dissociation between the velocity and position tasks suggests the cerebellar group was selectively impaired in velocity perception.

Effect of feature similarity on illusory conjunctions.

In four experiments, we examined whether the phenomenon of illusory conjunctions is constrained by feature similarity. Specifically, are illusory conjunctions more likely to occur between items with similar features than between items with dissimilar features? Feature similarity was manipulated in two dimensions: color and shape. Experiment 1 demonstrated that more illusory conjunctions occur between items with similar colors than between items with dissimilar colors.

Dissociation of short- and long-range apparent motion in visual search.

The visual search paradigm was used in four experiments to investigate apparent motion perception. The addition of distractor items led to a linear increase in reaction time under long-range (LR) conditions (greater than 35 min of arc displacement), whereas reaction time was independent of displays size under short-range (SR) conditions (less than 18 min of arc). Although clear performance differences were obtained, Ss had difficulty in distinguishing between the two types of apparent motion displays when asked to make such judgments (Experiment 2).

Illusory conjunctions inside and outside the focus of attention.

This article addresses 2 questions that arise from the finding that visual scenes are first parsed into visual features: (a) the accumulation of location information about objects during their recognition and (b) the mechanism for the binding of the visual features. The first 2 experiments demonstrated that when 2 colored letters were presented outside the initial focus of attention, illusory conjunctions between the color of one letter and the shape of the other were formed only if the letters were less than 1 degree apart. Separation greater than 2 degrees resulted in fewer conjunction errors than expected by chance.

The role of cerebellar structures in the execution of serial movements.

Thirteen patients with bilateral cerebellar disease and 12 patients with unilateral cerebellar disease were instructed to execute movement sequences in response to a simple reaction signal. Each to-be-executed sequence consisted either of a single, two, or three keypress components. Evidence for cerebellar involvement in the execution of programmed responses was sought in the pattern of response onset times and interkeypress times.

Timing functions of the cerebellum.

This study investigated the effects of different types of neurological deficits on timing functions. The performance of Parkinson, cerebellar, cortical, and peripheral neuropathy patients was compared to age-matched control subjects on two separate measures of timing functions. The first task involved the production of timed intervals in which the subjects attempted to maintain a simple rhythm.

Line segregation.

Six experiments investigated the preattentive segregation of line-like patterns composed of discrete elements in a background of distractors. The results indicate that other factors in addition to spatial density influence line segregation. Edge alignment, edge length and principal axis orientation also affect line segregation.

Tests of a temporal theory of attentional binding.

Different features of stimuli present in the field of view appear to be registered in different cortical maps. How, then, are the features that come from the same object bound together rather than mistakenly assembled with features coming from other simultaneously present objects? One theory supposes that an attentional mechanism intercepts input coming from particular retinal locations at a way station prior to parsing of the features from the same object. Any enhancement (or facilitation) at that stage will cause all the features from that object to be modified simultaneously in the downstream registers.

Dissociation of the lateral and medial cerebellum in movement timing and movement execution.

In a previous study (Ivry and Keele, in press), cerebellar patients were found to be impaired on both a motor and a perceptual task which required accurate timing. This report presents case study analyses of seven patients with focal lesions in the cerebellum. The lesions were predominantly in the lateral, hemispheric regions for four of the patients.

Force control and its relation to timing.

Previous work (Keele & Hawkins, 1982; Keele, Pokorny, Corcos, & Ivry, 1985) has suggested two general factors of coordination that differentiate people across a variety of motor movements, factors of timing and maximum rate of successive movements. This study provides comparable evidence for a third general factor of coordination, that of force control. Subjects who exhibit low variability in reproducing a target force with one effector, the finger, tend to show low variability with two other effectors, the foot and forearm.

Force and timing components of the motor program.

Three experiments were undertaken to assess the effects of variations of force and time on both simple and choice reaction time. The first two experiments demonstrated that although latency did not vary as a function of force, timing variations, such as requiring that a response be maintained, led to consistent changes in reaction time. These results led to the development of a model of motor programming in which force and timing are dissociated as separate components.