Neuropsychological Studies Of Category Learning

One approach that has successfully demonstrated implicit learning of categories is the use of data from amnesic patients. The fact that amnesic patients are able to exhibit normal category learning in several paradigms suggests that implicit learning can support normal performance. In addition to their intact performance on artificial grammar tasks described previously, amnesic patients have been shown to perform normally on category learning based on similarity to a prototype, category learning based on feature frequency, and category learning based on probabilistic associations. In each case, amnesic patients are severely impaired at recognizing training exemplars. Importantly, there are circumstances in which amnesic patients do not perform as well as normal subjects, particularly when there are a relatively small number of exemplars used during training that are highly memorizable. In those tasks in which amnesic patients are able to perform normally, it is likely that subjects are not basing their judgments on explicit comparisons to items stored in memory because the exemplars are not easy to memorize (letter strings and dot patterns) or a large number of exemplars are presented during training. These data further indicate that categorization can be accomplished using different kinds of knowledge depending on the task. It can be concluded that category learning does not necessarily depend on those medial temporal lobe structures that are required for explicit memory of exemplars.

If category learning can proceed independently of the brain system that supports explicit memory, it shows that other neural systems are capable of supporting this ability. Because there are multiple types of learned information that are used in different categorization tasks, it is highly likely that there is no single neural system involved in all category learning or even involved in all implicit category learning. Rather, different types of category learning are likely to depend on different brain systems. One system that has been linked with implicit learning is the basal ganglia. Damage to the basal ganglia as a result of Huntington's disease or Parkinson's disease impairs motor skill learning. Recent evidence suggests that the basal ganglia may be involved in nonmotor forms of implicit learning as well. These patients are impaired on classification tasks in which category membership is based on a nonverbal rule or a probabilistic association that is difficult to verbalize. For example, in the probabilistic classification task, a set of arbitrary cues (such as cards with geometric shapes) are probabilistically associated with one of two outcomes (such as rain or shine if a weather prediction cover story is used). Because of the probabilistic nature of the associations, memory for single trials is not very helpful for performing the task. Rather, information gleaned over multiple trials must be used to classify each instance. Patients with basal ganglia disorders exhibit severe impairments in this task. According to one model of implicit category learning, the connections from the striatum to the prefrontal cortex via the globus pallidus and thalamus are the basis for the associations between perceptual inputs and responses that are the basis for nonverbal rule learning. It may be that the striatum actually represents the rules, or that the rules just emerge as a property of these perception-action associations. These nonverbal rules are represented as associations between cues and responses ("If triangles appear, press sun key''). In contrast, for verbal rules that are learned declaratively, there is an explicit association between stimuli and outcomes ("If triangles appear, the weather will be sunny, so I should press the sun key'').

Another potential difference between basal ganglia-dependent category learning and learning that depends on the medial temporal lobe is the fact that the neostriatum may be the actual storage site for learned associations. In contrast, the role of the medial temporal lobe in explicit memory is time limited. Patients with medial temporal lobe damage are able to retain memories from long before their brain injury, demonstrating that the eventual storage sites of these memories are not in the medial temporal lobes. In models of neostriatal function, memories are stored in the connections within the basal ganglia or in the connections within the basal ganglia and thalamus. These models predict that damage to the basal ganglia would disrupt not only new nonverbal category learning but also previously learned nonverbal rules. These patients would not necessarily be impaired at categorizing stimuli based on previously learned explicit rules based on semantic knowledge (sorting fruits and vegetables into separate categories). However, there is some indication that these patients have difficulty with some previously learned implicit rules pertaining to language and social cognition, two domains in which rule learning appears to proceed implicitly.

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

Get My Free Ebook

Post a comment