Topic 3: Visual Object Recognition Flashcards
What does the term agnosia mean?
refers to a broad range of deficits related to recognition
What is visual agnosia?
visual agnosia involves deficits in visually-based object recognition, which occur in the absence of other accompanying problems with lower-level visual processing (i.e., those affected still have intact visual sensation)
this condition gained widespread attention with the publication of Oliver Sacks’ book “The Man Who Mistook His Wife for a Hat”
How does visual agnosia highlight the hierarchical nature of visual processing?
we go through different stages of processing to recognize objects
problems can occur at any of these stages
the particular stage(s) at which visual processing “breaks down” in an individual has implications for how that impacts their perception
the fact that visual agnosia presents without accompanying problems in lower-level visual processing suggests the problem occurs “upstream” (i.e., farther in the sequence of processing) from V1 in visual cortex
What is the area of damage in visual agnosia?
many cases involve known temporal lobe damage, representing a contribution to our understanding of this phenomenon from neuropsychological evidence
What are the two major types of visual agnosia?
apperceptive and associative
What is apperceptive agnosia?
typically involves an inability to recognize familiar objects based on shape
one way to think about this type of agnosia is as primarily affecting one’s ability to form a coherent mental representation (i.e., percept, or pattern) of whatever stimuli is being visually processed, almost as though they can process/represent the pieces but can’t “put the pieces together”
can also involve a degraded ability to recognize objects under certain conditions (poor lighting, non-canonical viewpoints, etc.)
may primarily affect earlier stages of visual processing more closely related to basic sensation processing (e.g., lower-level visual properties)
What is global vs. local processing in apperceptive agnosia?
apperceptive agnosia patients often have difficulty focusing on the whole (the global level) and tend to only draw meaning from small portions of whatever stimuli is being processing (at the local level)
What is a Navon figure?
can be used to test for impairments in local or global processing
local level: the stimuli is made up of many small number 4’s
global level: the many small number 4’s make up a larger pattern (a single number 7)
What is the lateralization related to global vs. local processing?
global features tend to preferentially processed when stimuli are presented to the left visual field
local features tend to be preferentially processed when stimuli are presented to the right visual field
in addition to hinting at some lateralization effects, this also provides a useful experimental method that can be applied in various ways to manipulate/isolate effects related to local or global processing
What is associative agnosia?
involves an inability to connect visual perceptions with meaning/memory
in contrast to apperceptive agnosia, seem to be less about difficulty extracting information about shape/form from coherent mental representation (i.e., percept, or pattern)
patients with this type of agnosia are capable of “putting together the pieces” of a stimuli to form a coherent mental representation (i.e., pattern, perception, etc.) of whatever they’re visually processing but then can’t connect that with any meaning
may primarily affect later stages of visual processing related to perception (higher-level meaning)
What is the performance of copying model drawings tasks of people with apperceptive agnosia?
are pretty much entirely unable to copy the model objects
this speaks to the difficulty they have extracting information from shape and form to produce a coherent mental representation that captures the essence of what they’re looking at
in contrast to the patients with associative agnosia, their drawings very clearly stand out as looking “abnormal”
What is the performance of copying model drawings tasks of people with associative agnosia?
they can copy the form of the object very accurately
however, their success often relies on a strategy meant to compensate for their deficit and they tend to copy the drawings by meticulously duplicating exactly what is on the page line by line, rather than incorporating elements based on firsthand experience with the objects (as someone without agnosia might do)
as a point of comparison, someone without visual agnosia might use the model picture as a reference point yet still draw an object based on (at least in part) their own prior knowledge of what it looks like in the real world
what they are doing is more akin to using a “grid method” for drawing
What are some other subtypes of visual agnosia?
various “subtypes” of visual agnosia have been identified, involving more selective/specific deficits (e.g., color agnosia, in which individuals cannot identify colors of common objects)
agnosia in other modalities can also occur (e.g. auditory agnosia, tactile agnosia, etc.)
What is anomia?
patients with agnosia sometimes also demonstrate impairments in object naming (anomia), though this can also happen independent of typical visual agnosia symptoms
among other things, this suggests that retrieving verbal labels also represents a somewhat dissociable stage of processing related to visual perception
What is category specific visual agnosia?
these involve more selective impairments in identifying objects belonging to a particular category (tools, living things, etc)
What is prosopagnosia?
the most well-studied example of a category specific form of visual agnosia is prosopagnosia, which involves a selective agnosia for faces
What is Capgras syndrome?
involves a delusional belief that someone in their life (usually someone important, e.g., friends, partners, family members) has been replaced by an “imposter” or “doppelganger”
can cooccur with various forms of pathology (brain damage, dementia, schizophrenia, etc.)
on some level, this involve a deficit in object/person identification and can therefore be likened to various forms of visual agnosia
at the same time, the fact that those affected can still perceive the people as at least resembling the persons true identity implies rather different underlying mechanisms than visual agnosia
What are the underlying mechanisms of Capgras syndrome?
there seem to be (at least) two distinct neural pathways that mediate recognition of familiar faces: one conscious/explicit and one unconscious/implicit
helps explain why some people with prosopagnosia, while being unable to explicitly identify faces, may nevertheless show some sign of implicit recognition using galvanic skin response (GSR)
we see the inverse of this with some Capgras patients: intact conscious/explicit processing but impaired unconscious/implicit processing
What brain regions may be relevant for understanding Capgras syndrome?
Ramachandran (1998) hypothesized that abnormal (impaired) connectivity between the inferotemporal cortex and the amygdala may be responsible
more generally, various differences in connections between the temporal lobe and limbic system may contribute
given the role played in memory for structures in/around the medial temporal lobe (such as the hippocampus), it has been speculated that deficits in the retrieval of episodic memories in particular may also be involved
What is reduplicative paramnesia?
disorder in which people believe that a familiar location has been duplicated
those affected may believe they are in an exact copy of the original location that is nevertheless not the same place (as it is claimed to be)
this disorder seems to occur after frontal lobe damage, suggesting a potential role for the same region in Capgras delusion
this also seems consistent with observations that Capgras-like symptoms are sometimes observed in various forms of dementia, which can also affect frontal areas
What is Fregoli delusion?
representing a sort of “mirror image” of Capgras syndrome, Fregoli delusion involves the belief that multiple individuals who are presented as distinct people are in fact the same person
this can manifest as thinking that someone has “transformed” themselves into another form, adopted a disguise, and/or are able to occupy multiple bodies
someone with Fregoli delusion may believe this (singular) person is transforming in order to trick them, manipulate the, do them harm, etc., and often involves paranoid thoughts revolving around persecution
What areas of the brain are involved in Fregoli delusion?
most often associated with damage to frontal and temporal-parietal areas
What are the underlying mechanisms of Fregoli delusions?
researchers have hypothesized that abnormal activity in memory-relevant regions may contribute to these delusions
whereas there may be an absence of normal memory activation in Capgras, there may be an abnormal (and “inappropriate”) activation of representations in memory that come to be associated with stimuli that are actually unrelated
there is some evidence for reduced inhibition in case studies of this condition, as evidenced by deficits in attention ad executive function, which could be consistent with the idea of abnormal/inappropriate activation
How are dopaminergic systems involves in Fregoli delusions?
we know that “overactivity” of dopaminergic systems can be associated with delusions in various situations (schizophrenia, amphetamine-induced psychosis, etc.)
accordingly, there is also evidence that delusional symptoms can fluctuate across the course of levodopa (L-DOPA) treatment, increasing in frequency/severity as dopamine concentrations rise (providing a sort of experimental, i.e. non-correlational, model)
it has therefore been hypothesized that abnormal dopamine activity may sometimes contribute to Fregoli delusions, and/or it may actually be brought on by dopamine-based treatments
What are inversion effects?
are said to occur for stimuli that people have difficulty noticing unusual details (anomalies) in which turned upside down yet are easily and rapidly detected when the stimuli is right side up
inversion effects are strong for faces, which has been argued to indicate they are processed holistically (rather than in a “piecemeal” or serial way, a little at a time/bit by bit, etc.)
What is type identification?
higher-level (superordinate) category e.g., dogs, cats, faces, houses, etc.
this is what we generally think of as being affected in visual agnosia
What is token identification?
specific examples from a given category, e.g., their dog, my cat, your face, etc.
this is what seems to be affected in prosopagnosia
Who presented that the idea that the fusiform face area (FFA) play a major role in face recognition?
Kanwisher is closely associated with a line of work arguing that the fusiform face area (FFA) is specialized (calibrated, or “tuned”) for processing face-like stimuli
much of her early work focused on distinguishing processing related to type vs. token identification, which may help explain the perspective she brought to bear on understanding what appears to be the selectivity of the FFA
What are some of the common debates surrounding the fusiform face area?
given that many people demonstrate FFA activation when viewing faces, another question is whether FFA activation causes us to perceive faces, or whether it’s activation is merely correlational
along similar lines, what other parts of the brain may be important for facial processing?
another debate in the literature is whether or not the function of the FFA is “intrinsic” (e.g., coded for in our genes), or dependent on experience
What is electrocorticography (ECoG)?
records electrical signals directly from the cortex surface
this is in contrast to trying to pick up the remnants of those signals by placing the electrodes directly on the scale, as with EEG
ECoG is often used in clinical contexts (e.g., to localize tissue that is especially prone to epilepsy)
can provide a unique research opportunity to measure neural activity in a way that isn’t usually possible
can also be used to electrically stimulate the cortex (which also affords some unique research opportunities)
What did the Parvizi et al. (2012) study measure using ECoG in the FFA?
Parvizi et al. (2012) measured a signal that is thought to be closely related to the OLD signal (oscillatory power in the Gamma band) and, like that signal, reflect higher rates of synaptic activity
they found similar results as with fMRI (e.g., elevated activity in the FFA when viewing faces)
they were able to directly stimulate the FFA and observe (in real time) what impact that had on the perceptual experience of the patient fitted with the ECoG
the patient reported that stimulation of the FFA resulted in a distortion to his perception of faces in the room (but nothing else), supporting the idea this region plays a causal role in the perception of faces
What are variations in dissociations among prosopagnosia patients?
one case study in which the patient could recognize facial expressions but not identify faces (e.g., recognize who they belonged to)
another case study in which a patient could identify faces but not facial expressions (would we expect a double dissociation related to this if we obtained the right imaging data)
What are variations in within-category impairments among prosopagnosia patients?
case study of a birdwatcher who lost the ability to discriminate between different species of birds, without having their perceptual identification of objects from other categories obviously affected
How does the idea of perpetual expertise relate to the fusiform face area?
although deficits in face perception may be the most likely/obvious symptom, what we refer to as prosopagnosia may more generally reflect a loss of perpetual expertise, which may take various different forms (even if the most obvious as it applies to facial processing for the average person)
What is the expertise hypothesis of the fusiform face area?
an alternative kind of account to the idea that the FFA is specialized for processing faces is that it is more generally used to process any stimuli that one develops expertise in within the visual domain
the average human probably becomes very good at recognizing faces due to the value of being able to do so in a highly social society (remembering who has helped/hurt you in the past etc.)
What is one advantage of the expertise hypothesis of the fusiform face area?
it seems entirely consistent with the variability we often see in case studies (e.g., bird watcher losing the ability to discriminate amongst different species after FFA damage)
What is a potential confound that could arise in a fMRI study that tries to prove the expertise hypothesis?
could relate to the way the participants naturally engage with the stimuli
if we’re shown pictures of faces with minimal instructions, the average person may (relatively automatically/spontaneously) attempt to determine whether they know the person
on the other hand, although we could go through the same recognition process when shown other kinds of stimuli if we wanted to, it may be the case that the average person is much less likely to spontaneously make that kind of judgment for other categories of stimuli
if so, the way participant approach the task may potentially differ across stimuli types, which could introduce a confound
How could we test the expertise hypothesis?
perhaps the most well-known attempt is often referred to as the “Greebles” study, named after the novel stimuli used that was created specifically for the experiment
by creating stimuli that didn’t already exist, Gauthier and Tarr (1997) effectively controlled for prior exposure
What is the “Greebles” study by Gauthier and Tarr (1997)?
the Greeble stimuli show inversion effects, which suggest they are processed in a holistic way (just like faces: we don’t focus on one part at a time in isolation in a “piecemeal” fashion)
they tend to be categorized at the individual/token level, rather than category/type level (once again, like faces)
before “training” (to recognize/categorize the Greeble stimuli), the FFA does not respond strongly to Greeble stimuli
after many hours of training, the FFA ow has a robust response to those stimuli
suggests acquiring perceptual expertise with these stimuli led to recruitment of the FFA
What is one thing that could be problematic about the Greebles study?
could the fact that they chose a novel stimulus that, although not existing before their study, nevertheless looked a bit like they had faces (and/or “behaved like face stimuli” in various ways) be problematic?
once you put a meaning on a abstract thing, it’s hard to not think of it that way
What happened when the Greebles study was replicated with a prosopagnosia patient?
interestingly, Bukach et al. (2012) replicated the above experiment with a prosopagnosia patient and found they were eventually able to reach “expert” status (albeit they required more trial than controls to achieve this)
they were thought to use alternative strategies to compensate for their deficit, as we’ve discussed happens in other contexts (e.g., visual agnosia)
What is the problem of multiple comparisons?
while not correcting for multiple comparisons can increase the rate of false positives (Type 1 error), correcting for multiple comparisons can also make the threshold so high for achieving statistical significance that it becomes very likely that you will be unable to reject the null hypothesis even if it is not true (Type 2 error, or a false positive)
What is the Bonferroni correction?
at the level of a whole brain, which typically has >20,000 voxels, one would expect that 1000 or more voxels would be falsely identified as being “activated” unless one were to apply a correction
the most rigorous is to divide each p value by the number of tests (called Bonferroni correction)
for just two voxels, this means that a critical p of 0.025 needs to be achieved; not a big deal; but for 20,000, the critical p would drop to 0.00000025
How was Kanwisher able to get around the problem of multiple comparisons when studying the FFA?
Kanwisher’s work was able to get around the problem of multiple comparisons by using a approach in which, instead of doing many comparisons involving the activity of individual voxels at singular points in times OR correcting for multiple comparisons, she essentially averaged the activity of specific groups of voxels over many trials
What is the logic of ruling out alternative explanations?
if we find that region 1 responds more strongly to object A than object B, would you feel comfortable making the claim that must mean region 1 is specialized for object A? probably not
a much stronger argument could be made if we found that region 1 responds more strongly to object A, as compared to a large array of other kinds of objects (object B, object C, object D, etc.)
so we can’t rule out the idea the region 1 responds to object A and another given object (e.g., object X) until we directly compare activation of region 1 when shown object A, with what happens when shown object X
Why is it hard to use the logic of ruling out alternative explanations when studying the fusiform face area?
we can’t get close to testing every possible comparison we could think of (with every other kind of object that exists in the world)
we therefore have to practical and at some point decide the balance of evidence is enough to support a claim like the FFA is specialized for responding to faces, without going overboard, but with enough data to feel reasonably confident
so you can really never be absolutely certain any given region ONLY responds to one type of stimuli and never anything else but it’s important to gather as much evidence as you can to at least minimize the possibility that there is some obvious alternative explanation (e.g., another kind of major category of objects that you didn’t think to test)
What is one way Kanwisher tried to be as thorough as possible when studying the FFA?
involved a very “low-level” solution, simply bring the experimental participants back for multiple fMRI sessions so they could gather enough data, to rule out enough alternative explanations, to satisfy the scientific community that her argument was at least plausible (albeit not an absolute certainty)
What is multi-voxel pattern analysis (MVPA)?
in theory, this allows us to go beyond simply looking for “hot spots” of activation (i.e., what part of the brain become most active when viewing stimuli x) to examine the distributed pattern of activation throughout the whole brain, which is presumably important in many situations
What is the difference between fMRI and MVPA?
we might think of traditional “univariate” fMRI studies as being about measuring localization of function, while MVPA focused more on distributed representations
What were the within-category correlations found in the Haxby et al. (2001)?
e.g., how does the distributed pattern of activation when viewing faces in one subset of the data (even numbered trials) compare to viewing faces in another subset of the data (odd numbered trials)?
What were the between-category correlations found in the Haxby et al. (2001)?
e.g., how does the distributed pattern of activation when viewing faces in one subset of the data (even numbered trials) to viewing tools in another subset of the data (odd numbered trials)?
What were the results of the Haxby et al. (2001) study?
they found a high degree of within-category correlations (and low degree of between-category correlations), for all categories
on the surface, this would seem to suggest that there is a great deal of consistency in the activation of a widespread and distributed pattern of activation throughout the brain when viewing stimuli from a specific category
in other words, yes the FFA becomes active when you view faces, but so do lots of other areas… and that exact pattern of distributed activity is very consistent across blocks when viewing faces
at the same time, that distributed pattern (when viewing faces) seems to be rather different from what occurs when viewing tools
In the Haxby et al. (2001) study did the results change when the FFA was removed from the analysis?
all object-selective cortex: included data from all voxels
excluding maximally responsive voxels: excluded voxels with the strongest activation, or in other words, exclude “hot spots”
the same general result held, even when excluding the “hotspots” in the second version of their analyses: a high degree of within-category correlations, and low degree of between-category correlations, for all categories
What is the interpretation of the results from the Haxby et al. (2001) study?
the results of this study suggest that there is much more to facial processing than just the FFA
rather, a large/widespread network of regions seems to be recruited when viewing faces
along these lines, something similar could be said about the widespread/distributed pattern of activation when viewing objects from other categories
however, it is worth noting that simply excluding the “hot spots” does not control for signals those locations may be sending to the rest of the cortex
What are the specialized modules in/around the ventral stream that are important for object identification of various kinds?
fusiform face area (FFA): selectively responds to faces (maybe?)
parahippocampal place area (PPA): selectively response to places/scenes/landscapes
extrastriate body area (EBA): selectively response to body parts
What deficits are associated with damage to the PPA?
damage to the PPA is associated with difficulty recognizing places/scenes/landscapes (topographic agnosia) providing a parallel of sorts with prosopagnosia
