Vision

1. Vision Early and late visual pathways What is visual agnosia? Types of visual agnosia Cognitive models Category specificity? Summary

2. Occipital lobe and vision

3. The occipital lobe • The occipital lobe is involved in recognition of visual sensory input (colour, movement, line orientation, texture, light and shade). • Higher vision: face, object and word recognition. • Specialises in the parallel analysis of incoming information (e.g., object, face, word recognition).

4. Vision • The occipital lobe at the back of the brain receives information from the eye (retina) and transmits this to the optic radiations via LGN. • The visual field is divided into halves with the contents of the left visual field projected to the right hemisphere and the contents of the right visual field projected to the left hemisphere. • Lesions to the visual pathways yield deficits called hemianopias (left and right) and quadrantanopias (superior and inferior).

6. Lower visual pathways • Non-striate vision via the Superior Colliculus. • Movement, location (e.g., Blindsight patient DB). • Spatial processing (e.g., Patient DF - actions intact). • Primitive vision (e.g., Schnieder, 1967). • Striate vision via the geniculo-striate pathways. • Object recognition (e.g., perception and meaning). • Face recognition (e.g., features & person identity). • Higher order visual cognition (e.g., imagery).

7. Blindsight • Patients (DB) can perform visual tasks in the absence of any conscious visual perception. • Detection of location, movement, form and colour can occur in the scotoma - damaged visual area. • Damage is to the geniculo-striate pathway but the non-striate pathway may be intact. • Suggests a form of vision without awareness. • A ‘pre-cortical’ visual system supporting a primitive form of visual discrimination that occurs via the superior colliculus (in the pons).

8. Higher visual pathways • What pathway (located in the temporal lobe) • Visual perception-to-meaning processing. • Object constancy: features or principal axis? • Global and/or local processes in object recognition? • Categories of knowledge about objects? • Where pathway (located in the parietal lobe) • Perception-to-spatial location processing. • Conscious and automatic visual processing? • ‘What’ and ‘where’ or ‘what’ and ‘how’? • Unitary or modular action-semantic system?

10. ‘What’ pathway impairments • Impaired recognition of objects (visual agnosia). • Impaired recognition of faces (prosopagnosia). • Impaired recognition of words (dyslexia). • Issue: Categories of knowledge? • Living versus nonliving things. • Sensory versus functional knowledge. • Modular account e.g. visual-verbal semantics. • Organised unitary contents hypothesis OUCH!

11. Agnosia • Agnosia is characterized by an inability to recognize objects despite having intact knowledge of the object’s characteristics. • Agnosics may have difficulty recognizing the geometric features of an object or they may be able to perceive the geometric features but not know what the object is used for. • Agnosia can be present in other sensory modalities e.g., hearing (auditory agnosia).

12. Types of agnosia • Sensory agnosia (Warrington, 1985): • Impairment to colour, form, acuity, motion detection. • Apperceptive agnosia (Lissauer, 1890): • no visual percept is formed. • impairment is to object perception. • Associative agnosia (Lissauer, 1890): • Perception is intact and impairment is to the association of the percept with meaning. • Knowledge about objects can be impaired. • Category specificity (living versus non-living).

13. Apperceptive visual agnosia • Patient cannot point to named objects. • Patient is unable to match, copy, or discriminate simple visual forms (geometric shapes) but may draw well from memory. • Visual acuity (detection of spatial frequency of line gratings) is normal and perception of stimuli in both visual fields may be intact. • Recognition of objects may improve if the stimuli are moved around the environment.

14. Associative visual agnosia • Patient cannot point to named objects (as with apperceptive agnosia). • Patient is able to match, copy and discriminate simple visual forms such as geometric shapes (cf. apperceptive agnosia). • Patient is unable to process meaning because: • May be unable to access meaning from vision but recognise objects from touch (which implies that knowledge of the object is intact) - optic aphasia. • May have a deeper loss of knowledge about objects from any modality (semantic association problems).

15. Clinical Tests

20. Simultanagnosia (Luria, 1959). • Dorsal (Farah, 1990) • Patient is unable to see or attend to more than one object at a time. • Ventral (Farah, 1990) • Patient is unable to relate a small portion of an object to the rest of the stimulus. • Ventral simultagnosic patients are better at negotiating their environment than dorsal simultagnosics (i.e. action is unaffected).

21. Integrative agnosia • HJA (bilateral occipital lobe stroke) showed normal form perception and naming of object parts but impaired naming of whole objects. • He could copy drawings but is not associative agnosic because he could name object parts. • Two processes in object recognition: • Global form perception = intact for HJA. • Binding stimuli together = impaired “ “.

22. Other forms of visual agnosia • Category specific agnosia • Cannot name living things but can name nonliving things (or vice versa). • Colour agnosia • Cannot distinguish different colours. • Prosopagnosia • Cannot recognise faces. • Optic aphasia (Freund, 1889) • Cannot name visual stimulus but can point to it when named; can name object from touch; and can demonstrate its use.

23. Functional model of object recognition

24. The primal sketch • An initial representation that represents: • (1) brightness changes across the visual field; • (2) the two-dimensional geometry of the image. • Features such as edges produce abrupt intensity changes. • No actual perception of objects at this point.

25. The 2 1/2 D sketch • Sources of information concerning depth and location are assembled into a 2 1/2 D sketch or viewer-centered representation. • stereopsis • texture gradients • shading • A viewer centered representation - represents the spatial locations of visible surfaces from the viewer's position. • Can recognise objects but lacks generality.

26. The 3-D model representation • A three dimensional or object-centered representation which is independent of the viewer's position (unusual view of an object). • Specifies the real shape of objects and surfaces present in the visual field. • This is the point in the object recognition process at which the viewer centered representation is matched with stored structural descriptions of the object.

27. Campion & Latto (1985) • Patient RC: Carbon monoxide poisoning. • Effectively blind as could not detect small changes in brightness and also could not recognise, copy or draw geometric shapes (therefore similar to apperceptive agnosics). • BUT could negotiate objects, reach out for objects and comment on colour and texture. • Due to a disruption of the ability to construct a primal sketch (Ellis and Young, 1986).

28. Mr S • Could distinguish small differences in stimulus brightness and wavelength on psychophysical testing so able to form a primal sketch. • However, on tasks involving shape or form discrimination, Mr S was severely impaired. • Unable to recognise or copy objects, pictures of objects, body parts, letters, faces. • Impairment in constructing viewer-centered or 2.5D representation (apperceptive agnosia).

29. Warrington & Taylor (1978) • Showed patients with posterior injuries of the right hemisphere 20 photographs of objects taken from conventional and unusual views. • Few errors were made from a conventional view so patients were able to form a viewer-centered representation but they were unable to identify objects presented from an unusual viewpoint. • Patients are impaired at forming an object centred (3D) representation (though Shallice 1988 disagrees with this conclusion).

30. Clinical assessment • Birmingham Object Recognition Battery. • Distinguishing figure from ground. • Shape recognition. • Efron squares test. • Copying objects. • Drawing from memory. • Naming line drawings.

31. Category specific agnosia • Warrington and Shallice (1984) reported a patient called JBR who following an acute lesion to the left temporal lobe (as a result of herpes encephalitis) had a selective deficit when asked to name pictures from just one semantic category – living things.

32. Artefacts • By contrast JBR was able to name non-living objects very well including those with low frequency names such as ‘accordion’ that were matched for the number of letters in the name and the visual complexity of the object.

33. Double dissociation • Patient YOT (Warrington & McCarthy, 1987) was worse at naming nonliving things but was very good at naming large, nonmanipulable objects. • Suggests that the organisation of semantic memory might break down into taxonomic categoriesor reflect different properties of living (visual-sensory) and nonliving (functional) items. • LA (Gainotti & Silveri, 1996) was worse at living than nonliving things and was worse at giving visual information compared with functional information about both living and nonliving things.

34. Summary of patient data LivingNonliving Example cases AnimateInanimate Animal Fruit Artefacts x x √ (Warrington & Shallice,1984). √ √ x (Sheridan & Humphreys, 1993). x √ √ (Hart & Gordon, 1992). √ x x (Hillis & Caramazza, 1991). √ x √ (Hart, Berndt, Caramazza, 1985). x √ x Not reported

35. Categories of knowledge about objects? • Multiple semantic systems (Warrington/Shallice) • Patients who have no access to object knowledge from verbal input but can name pictures successfully. • Modality specific (visual and verbal) meaning systems. • Verbal semantic and visual semantic systems? • Unitary semantic system (Caramazza/Coltheart) • Different types of object depend on different types of encoding e.g., living things require featural encoding. • Knowledge only appears to fractionate because of differences in task difficulty (verbal & visual tasks).

36. Confounding factor accounts • Confounding factors for verbal/visual knowledge: • Pictures ‘afford’ more information than words. • Agnosia causes apperceptive visual problems. • Modality of input impairments cannot be excluded. • Confounding factors for living/nonliving things: • Visual familiarity, similarity, complexity (Funnell, 2000) • Animals are more structurally similar to each other than artefacts (Humphreys,et al, 1995; Funnell, 2000). • Monkeys discriminate nonliving things better than living things (Gaffan & Heywood, 1993).

37. Differential-weighting hypothesis • Category specific effects on recognition result from a correlated factor such as the ratio of visual versus functional features of an object • living more visual and nonliving more functional. • Farah & McClelland (1991) report a dictionary study showing the ratio of visual to functional features for living things and nonliving things: • living things was 7.7:1 and nonliving was 1.4:1.

38. Farah and McClelland (1991) • A single system with functional and visual features. • Model was trained to associate functional and visual features differently for living and non-living things. • Two different inputs (verbal and visual). • Model lesioned to either visual or functional units; • A) Visual units>living things; Functional units>nonliving things. • B) Visual units>impaired functional knowledge of living things. • Loss of both visual and functional knowledge about a living thing can occur in a unitary single system that only distinguishes between visual and functional attributes.

42. Thompson-Schill et al., (1999) • Interactive modality hypothesis predicts that questions about visual and functional semantic knowledge for living things will engage visual semantic processing. • Questions about the visual semantic attributes of nonliving things will also engage visual semantic processing. • Scaned the fusiform gyrus while asking questions about visual and functional properties of living and nonliving objects.

44. Thompson-Schill et al., (1999) • There was increased activity in the ventral (What) pathway - middle temporal gyrus - for all conditions suggesting a common system. • But questions about the visual semantic attributes of living and nonliving things engaged the fusiform gyrus differentially = an interaction. • Visual semantic retrieval may depend on both modality of input and the category of retrieval. • See Caramazza (2000) for a reply.

45. Funnell (2000) • Reported a patient with dementia (NA) who showed a category specific naming deficit: • not due to any of the confounding factors (visual complexity, similarity, familiarity). • not due to semantic problems. • resulted from an early visual problem possibly due to the access of stored structural descriptions. • conclusion is that category specific deficits can be a consequence of visual impairments in agnosia and do not necessarily reflect categories of knowledge.

46. Problematic cases • Does not explain the reverse dissociation e.g. living>nonliving (Sachett and Humphreys 1992). • Category specificity effect can be purely verbal • Is an apple round? (sensory) x • Is an apple edible? (functional) √ • Patients can show a nonliving>living dissociation but no difference in access to sensory/visual and associative/functional knowledge on testing. • Patients who are worse at perceptual knowledge tests but who show no category-specific deficits (for a review see Lambon Ralph et al., 1998).

47. Domain-specificity • Caramazza & Shelton (1998). • Unitary semantic system, which is split into three broad domains: animate objects, vegetation and artefactual objects. • Differentiation driven by evolution or perhaps by learning through Kohonen networks. • H1: No smaller category differences that these distinctions allow. • Thompson-Schill et al., (1999) report evidence from fMRI for category specific regions in the brain and interactivity between these regions.

48. Summary • Studies of visual agnosia have contributed to the identification of visual pathways in the brain confirmed using brain imaging methods. • Studies of visual agnosia have enabled us to develop theories of normal object recognition and identify necessary cognitive processes. • Studies of visual agnosia have shown that knowledge about objects can be dissociated into modular sub-systems.

49. Reading • Parkin Chapter 2&3. • Ellis & Young (Chapter 2). • Below I have listed some of the original specific cases reports about each type of visual agnosia as well as some recent case descriptions. Some readings are based on the box-and-arrow approach described in this lecture, and others take a different approach (eg neuroimaging, computational modeling). • * means basic reading; • ** more advanced; • *** very advanced.

50. Readings for category specificity • Caramazza, A. (2000). Minding the facts: A comment of Thompson-Schill et al’s “A special neural basis for category and modality specific semantic knowledge. Neuropsychologia, 38, 944-949. • Damasio, H., Grabowski, T.J., Tranel, D., Hichwa, R.D., Damasio, A.R. (1996). A neural basis for lexical retrieval, Nature, 380, 486-96. • Farah, M., & McClelland, J.L. (1991). A computational model of semantic memory impairment: Modality specificity and emergent category specificity. Journal of Experimental Psychology: General, 120, 339-357. • Forde, E.M.E., Humphreys, G.W. (1999). Category-specific recognition impairments: a review of important case studies and influential theories. Aphasiology, 13(3), 169-193. • Gaffan, D. and Heywood, C. A., (1993). A spurious category-specific visual agnosia for living things in normal humans and primates. Journal of Cognitive Neuroscience, 5, 118±128. • *Semenza, C. & Bisiachi, P. S. (1996). Warrington and Shallice’s (1984) category specific aphasic, J.B.R. In C. Code, C-W. Wallesch, Y, Joanette,& L.A. Roch. (Eds.). Classic cases in neuropsycholgy. Psychology Press. • *Thompson-Schill, S.L., Aquirrre, G.K. Esposito, M.D. & Farah, M.J. (1999). A neural basis for category and modality specificity of semantic knowledge. Neuropsychologia, 37, 671-676. • Warrington, E.K. & Shallice, T. (1984). Categories of knowledge: Further fractionations and an attempted integration. Brain, 107, 829-54.