Chapter 6: Object Recognition Flashcards
Agnosia
Book definition: “A neurological syndrome in which disturbances of perceptual recognition cannot be attributed to impairments in basic sensory processes. Agnosia can be restricted to a single modality, such as vision or audition. (p. 220)”
Agnosia is characterized by normally functioning sensory input – i.e. normal vision or hearing – but with a deficit in processing incoming sensory information.
Types of agnosia include:
- Apperceptive agnosia: the inability to accurately perceive sensory information.
- Associative agnosia: the inability to recognize or assign meaning to otherwise accurately perceived stimuli.
- Prosopagnosia: the inability to recognize faces.
- Alexia: the inability to recognize text.
- Visual-, auditory-, tactile agnosia: the inability to recognize visual, auditory, or tactile sensory information. These can be seen both as apperceptive and associative (see above).
Alexia
Book definition: “A neurological syndrome in which the ability to read is disrupted. Alexia is frequently referred to as acquired alexia to indicate that it results from a neurological disturbance such as a stroke, usually including the occipitoparietal region of the left hemisphere. In contrast, developmental alexia refers to problems in reading that are apparent during childhood development. The phrases are commonly used to indicate that reading is abnormal, either from a neurological disturbance or as part of development. (p. 240)”
(Text is slightly edited from the book, as I think Gazzaniga himself may have been suffering from a bit of sudden-onset alexia when writing it).
Analytical processing
Book definition: “Perceptual analysis that emphasizes the component parts of an object. Reading is thought to be a prime example of analytic processing in that the recognition of words requires the analysis of at least some of the component letters. Compare holistic processing. (p. 258)”
Apperceptive agnosia
Book definition: “A form of agnosia associated with deficits in the operation of higher-level perceptual analyses. A patient with apperceptive agnosia may recognize an object when seen from a typical viewpoint. However, if the orientation is unusual, or the object is occluded by shadows, recognition deteriorates. Compare associative agnosia. (p. 237)”
Agnosia refers to the general inability to process sensory information and exists in a variety of different forms.
Apperceptive agnosia, which can apply to senses such as vision, hearing or touch, is a failure in recognition due to a failure of perception. In apperceptive visual agnosia, patients usually experience a failure in recognition when presented with stimuli of limited information, e.g. objects pictured as simple line-drawings, seen from atypical viewpoints, or partially covered in shadow. In other words, there seems to be a deficit in object constancy.
The finding that apperceptive agnosia is more common in patients with posterior right-hemisphere lesions suggests that this hemisphere is essential for the operations required to achieve such object constancy.
Apperceptive agnosia is in contrast to associative agnosia, which is a failure in recognizing or assigning meaning to stimuli, even though they are accurately perceived – e.g. being able to correctly copy a drawing, but being unable to identify its meaning. This deficit is more common in patients with left-hemisphere lesions.
Associative agnosia
Book definition: “A form of agnosia in which the patient has difficulty linking perceptual representations with long-term knowledge of the percepts. For example, the patient may be able to identify that two pictures are of the same object, yet fail to demonstrate an understanding of what the object is used for or where it is likely to be found. Compare apperceptive agnosia. (p. 240)”
Agnosia refers to the general inability to process sensory information and exists in a variety of different forms.
Associative agnosia, which can apply to senses such as vision, hearing, or touch, is a failure in recognizing or assigning meaning to an otherwise accurately perceived stimulus. This deficit is often linked to lesions in the posterior left hemisphere.
Although patients often perform abnormally on tests of basic perceptual abilities (likely because their lesions not only affect a single area), they may be perfectly able to copy a drawing of an object, but unable to correctly identify the object itself.
This is in contrast to apperceptive agnosia, which is a failure in recognition due to a failure in the perception of stimuli – e.g. not being able to correctly copy a drawing due to an inability to perceive its shape. This deficit is more common in patients with posterior right-hemisphere lesions.
Category-specific deficits
Book definition: “Recognition impairment that is restricted to a certain class of objects. Some rare individuals demonstrate an impairment in their ability to recognize living things, yet exhibit near-normal performance in recognizing nonliving things. Such deficits are useful in the development of models about how perceptual and semantic knowledge is organized in the brain. (p. 243)”
Decoding
Book definition: “Using the brain activity, which is produced by a stimulus and detected by such methods as fMRI, in order to determine the original stimulus. (p. 261)”
This idea suggests that it should, at least in principle, be possible to analyze the brain’s activity and infer what a person is experiencing. In other words: mind reading.
Experiments on decoding have so far been able to produce simple encoding models, which use models of information processing in brain areas to predict or reproduce responses. Models like these have been able to fairly accurately predict whether a participant was shown a house or a face, as well as crudely reproduce presented visual stimuli (see picture). Researchers have also been able to predict a left/right response by participants up to several seconds before they were consciously aware of their own decision.
However, decoding the brain’s activity relies on several factors – e.g. the accuracy and resolution of both the equipment measuring it, and the current models of how the brain processes information. While fMRI and EEG can provide fairly detailed information to match current models, more precise equipment and models will have to be developed before more accurate predictions can be made.
Dorsal (occipital) stream
Book definition: “A processing pathway for visual stimuli that is specialized for spatial perception – for determining where an object is – and for analyzing the spatial configuration between different objects in a scene. (p. 222)”
The dorsal stream – also called the “where/how-pathway” – is involved with both the spatial configuration of objects as well as guiding actions needed to interact with them. In contrast, the ventral (occipitotemporal) stream – also called the “what-pathway” – is involved with object recognition.
The individual roles of the dorsal and ventral stream has been shown in the mail slot experiment, where participants have to match the orientation of a card to the orientation of a slot. Here, patients with damage to the ventral stream will be unable to identify “what” the visual aspects of the slot are, and fail at matching the card to its orientation. When asked to “mail a letter,” however, the still functioning action-oriented dorsal stream will know the “how” and effortlessly match the card to the slot.
Encoding model
Book definition: “A model of how information in the environment is represented. Encoding models are used in fMRI to predict the BOLD response to different stimuli. (p. 263)”
Extrastriate body area (EBA) / fusiform body area (FBA)
Book definition: “A functionally defined area in the lateral occipitotemporal cortex that has been found, in fMRI studies, to show a stronger response to images containing body parts relative to other animate and inanimate stimulus categories. (p. 258)”
Fusiform face area (FFA)
Book definition: “A functionally defined area of the brain, located in the ventral surface of the temporal lobe in the fusiform gyrus, which responds to selective stimuli, such as faces. (p. 249)”
While the name suggests a focus on faces, the fusiform face area has also been linked to general discrimination among highly familiar stimuli. Activation has been shown when asking experts on a certain subject to discriminate between items of that subject, such as cars or birds. However, this activation spreads across a much broader region of the ventral occipitotemporal cortex, indicating that the FFA may instead have a supporting role in object discrimination.
In addition to the FFA, the superior temporal sulcus also shows a higher activation for faces than for other stimuli. One theory suggests that while the FFA processes invariant facial properties (i.e. shape, size, and orientation), the role of the STS is to process certain types of dynamic facial information, such as emotion and attention.
Fusiform gyrus
Book definition: “A gyrus located along the ventral surface of the temporal lobe. This area has been shown in neuroimaging studies to be consistently activated when people view face stimuli. Neurological lesions that include the fusiform gyrus are associated with prosopagnosia, although the damage also extends to other regions of the cortex. (p. 249)”
Gnostic unit
Book definition: “A neuron or small set of neurons tuned for a specific percept (e.g., an apple). The concept of the gnostic unit is based on the idea that hierarchical models of perception imply that, at higher levels in the system, neurons become much more selective in terms of what they respond to. (p. 234)”
The idea of a gnostic unit is also the basis for the grandmother-cell hypothesis – i.e. the notion that there might be a specific cell which only fires when exposed to a distinct object, person, or scene.
Should there only be one such cell or group of cells, however, it poses several problems. For instance, should the gnostic unit stop responding, would we experience a sudden loss of the representation for “grandma”? And how would such a unit adapt to changes in the representation over time, such as grandma getting a haircut?
An alternative explanation is the ensemble hypothesis, which suggests that recognition is facilitated by the collective activation of many units. This would also explain how we can recognize similarities between objects and may confuse similar objects, as similar objects may share similar neuronal units. In addition, while losing some units would degrade recognition, the remaining units might still suffice.
Holistic processing
Book definition: “Perceptual analysis that emphasizes the overall shape of an object. Face perception has been hypothesized to be the best example of holistic processing, in that the recognition of an individual appears to reflect the composition of the person’s facial features rather than being based on the recognition of the individual features themselves. Compare analytic processing. (p. 258)”
Integrative agnosia
Book definition: “A form of agnosia associated with deficits in the recognition of objects due to the failure to group and integrate the component parts into a coherent whole. Patients with this deficit can faithfully reproduce drawings of objects; however, their percept is of isolated, unconnected parts or contours. (p. 239)”
Lateral occipital complex/cortex (LOC)
Book definition: “A region of extrastriate cortex that is part of the ventral pathway. Processing in LOC is essential for shape perception and recognition. (p. 227)”
Object constancy
Book definition: “The ability to recognize invariant properties of an object across a wide range of contexts. For example, although the size of the retinal image changes dramatically when a car recedes in the distance, our percept is that the car remains the same size. Similarly, we are able to recognize that an object is the same when seen from different perspectives. (p. 230)”
Optic ataxia
Book definition: “A neurological syndrome in which the patient has great difficulty using visual information to guide her actions, even though she is unimpaired in her ability to recognize objects. Optic ataxia is associated with lesions of the parietal lobe. (p. 227)”
Patients suffering from optic ataxia often appear to grope about like someone trying to find the light switch in a dark room. Although patients can accurately perceive and describe both objects and their orientation, they fail to apply the spatial knowledge when guiding movement.
A famous example of the effects of this syndrome is the mail slot experiment, where patients showed great difficulty matching the orientation of their hand to the orientation of a slot. This experiment is also used to demonstrate the difference between the ventral (“what”) stream and the dorsal (“where/how”) stream. Here, patients with damage to the ventral stream will be unable to identify “what” the visual aspects of the slot are, and fail at matching a card to its orientation. When asked to “mail a letter,” however, the still functioning action-oriented dorsal stream will know the “how” and effortlessly match the card to the slot.
Parahippocampal place area (PPA)
Book definition: “A functionally defined area of the brain (usually with fMRI), located in the parahippocampal region of the temporal lobe that shows a preferential response to stimuli depicting scenes or places. (p. 258)”
The parahippocampal place area appears heavily involved in the encoding and recognition of environmental scenes and may have developed as a way for early humans to remember where food supplies were located.
Lesions in the PPA can lead to an inability to recognize scenes, even though patients can recognize individual objects in the scene. Stimulation of the region has also been shown to produce intense visual hallucinations of places and situations.
Repetition suppression effect
Book definition: “The phenomenon seen during functional MRI in which the BOLD response to a stimulus decreases with each subsequent stimulus repetition. (p. 231)”
An fMRI study from 2002 used the repetition suppression effect in order to explore the hypothesis that the brain uses both view-dependent and view-invariant frames of reference to support object recognition – i.e. identifying objects by recognizing previously observed viewpoints, or by recognizing previously observed components of an object regardless of viewpoint.
In the experiment, a repetition suppression effect was observed in the left ventral occipital cortex, regardless of whether the object was shown from the same or a different viewpoint, supporting a view-invariant representation. However, the same suppression effect was only observed in the right ventral occipital cortex when the second presentation was from the same viewpoint as the first, supporting a view-dependent representation.
Ventral (occipitotemporal) stream
Book definition: “The visual pathway that traverses the occipital and temporal lobes. This pathway is associated with object recognition and visual memory. (p. 222)”
The ventral stream – also called the “what-pathway” – is involved with object and visual identification and recognition. In contrast, the dorsal (occipitoparietal) stream – also called the “how/where-pathway” – is involved with both the spatial configuration of objects as well as guiding actions needed to interact with them.
The individual roles of the dorsal and ventral stream has been shown in the mail slot experiment, where participants have to match the orientation of a card to the orientation of a slot. Here, patients with damage to the ventral stream will be unable to identify “what” the visual aspects of the slot are, and fail at matching the card to its orientation. When asked to “mail a letter,” however, the still functioning action-oriented dorsal stream will know the “how” and effortlessly match the card to the slot.
Visual agnosia
Book definition: “A failure of perception that is limited to the visual modality. In visual agnosia, the patient is relatively good at perceiving properties such as color, shape, or motion yet cannot recognize objects or identify their uses. (p. 220)”
