VISION PDF: The Student's Guide to Cognitive Neuroscience

# The Student's Guide to Cognitive Neuroscience ## Action Also Involves Selection - We can perceive multiple things at the same time, but our body prevents us from doing multiple actions at the same time. Perception → Attention → Action involves a shift from parallel to serial processing (i.e. se...

# The Student's Guide to Cognitive Neuroscience ## Action Also Involves Selection - We can perceive multiple things at the same time, but our body prevents us from doing multiple actions at the same time. Perception → Attention → Action involves a shift from parallel to serial processing (i.e. selection from many to one) In reverse, Action → Attention → Perception acts like a filter of sensory flow. ## The Spotlight Metaphor of Attention - Spotlight may move from one location to another. - It may zoom in or out (narrow or wide "beam"), e.g. if attending to words or attending to central letter in a word. - Location of attention not necessarily same as eye fixation ("looking out corner of one's eyes"). - Spotlight metaphor shouldn't be taken too literally (e.g. possible to split attention between 2 non-adjacent locations). ## Non-Spatial Attentional Selection - Object-based attention: a non-spatial 'spotlight'. - Different parts of visual ventral stream (representing different stimulus types/properties) increase in activity when attended (e.g. Kanwisher). - Same physical stimulus triggers different level of brain response depending on attentional relevance. ## Attention & Action: the Dorsal Stream - Parietal lobes specialized for spatial processing and have been called the "where" route (Ungerleider & Mishkin). - Parietal lobes also bring together different types of spatial representation that are needed for action (e.g. integrating visual space with body space) so also called the "how" route. ## Can Visual Agnosic Patients Act on Objects That They Can't Recognize? - Visual agnosic patients can act on objects that they can't accurately perceive. - Patient DF (Milner et al., 1991; Goodale et al., 1994). ## Recognizing Objects But Not Acting on Them - Optic ataxia = Poor visual guidance of reaching (parietal lobe damage, dorsal stream). ## Different Types of Spatial Representation - Brain doesn't regard space as a continuous single entity. - Space in the brain exists in many forms: - Locations on sensory surfaces (e.g. the retina; retinocentric space) - Location of objects relative to the body (egocentric space) - Location of objects relative to each other (allocentric space) - Finding the nearest bank and picking up a pencil both require spatial processing, but the computational demands and the type of spatial representation used are likely to be very different. ## The Rubber Hand Illusion (RHI) - Three different spatial maps are relevant - Visual map (where the [rubber] hand is seen) - Tactile map (where the body is touched) - Proprioceptive map (the true location of the hand) - There is a conflict between the visual and the proprioceptive maps and the brain has to decide 'who to believe'. - Vision wins! ## The Basic Problem - Receptor surfaces (e.g. on skin, retina) are constantly changing their location. - A Two Part Solution = - (1) We need to know the spatial arrangement of our body to locate external events in space (e.g. proprioception). - (2) We need to link together different maps of space ('sensorimotor transformations' or 'remapping'). ## Sensorimotor Transformations in the Brain - Spatial-sensory and motor information converges on the parietal lobes enabling coordinate transformations between different maps. ## How do Neurons in Dorsal Stream Enable Objects to be Acted On? - Separate neurons for reaching or looking. ## Parietal Reach Region (PRR) - Damage to this region is linked to optic ataxia (problems in visually-guided reaching). - Neurons respond to direction of arm movements relative to gaze (note that motor cortex responds to arm movements irrespective of gaze). - Parietal regions 'decide' whether to move eyes (LIP) or hands (PRR): intention as well as attention because both involve selection. ## Lateral Intra-Parietal Area (LIP) - Single cell electrophysiology shows... - Has motor properties (generates saccades) - Has sensory properties (both sound and vision), enables sounds to be remapped to eye-centered coordinates - Important for attention because... - Doesn't respond to all sensory stimuli (sparseness) - Responds more to stimuli that are unexpected such as sudden flash (i.e. important for bottom-up attention) - Responds more to stimuli that are task relevant (i.e. important for top-down attention) - Can enable covert and overt orienting (with/without eyes) - Codes a spatial 'salience map' (with left and right hemispheres having different biases) ## How do Neurons in Dorsal Stream Enable Objects to be Acted On? - Transformations along dorsal stream from gaze-centered neurons (LIP, PRR) to neurons with receptive fields centered on the body (VIP, premotor). ## From Attention to Action - Note that these parietal regions are not responsible for the actual movement of the body or the eyes - This is done by primary motor cortex (body) and frontal eye fields (eyes) - But frontal and parietal regions communicate in both directions ## From Action to Attention - Strong stimulation to frontal eye fields causes the eyes to move - Weak stimulation of frontal eye fields causes attention to shift - So shifting attention is like a non-executed action! ## Hemispheric Differences in Spatial Attention - Right hemisphere lesion has substantial effect. ## Neglect as a Disorder of Attention and Not Low-Level Perception - Neglect patients still activate visual regions in occipital lobes that they claim not to be aware of. - They are often able to detect objects on the left if cued there. - Affects auditory and tactile judgments as well as vision (e.g. sounds on left are mislocalized but still heard). - Phenomenon of visual extinction suggests different perceptual representations are competing for attention (and visual awareness). ## Neglect as a Disorder of Attention and Not Low-Level Perception (cont.) - Phenomenon of visual extinction suggests different perceptual representations are competing for attention (and visual awareness). ## What Happens to Neglected Information? - Ventral stream may continue to process neglected objects up to the stage of object recognition and possibly semantics. - Degraded line drawings presented in neglected field show subsequent priming effects when attended (Vuilleumier et al.). - Burning house experiment (Marshall & Halligan, 1988). ## Different Spatial Reference Frames: Evidence from Neglect - Double dissociation between perceptual and representational neglect suggests different spatial reference frames for external versus imagined (mind's eye) space. - Piazza del Duomo experiment (Bisiach & Luzzatti, 1978). ## Different Spatial Reference Frames: Evidence from Neglect (cont.) - Double dissociation between near and far space. - Near space = tested with line bisection using pen and paper. - Far space = tested with line bisection using projected image and laser pointer. - "Near" defined as within reach; if patient is given a stick rather than a pointer then "near" deficit extends into far space. ## Different Spatial Reference Frames: Evidence from Neglect (cont.) - Double dissociation between personal (bodily) space and near space. - Body neglect = failure to groom left of body (fluff test) or notice position of limbs. - Near space neglect = visual search of array of external objects. ## Different Spatial Reference Frames: Evidence from Neglect (cont.) - Some neglect patients attend to objects on the left side of space but omit to attend to one half of the object itself (object-based neglect). - Forms a double dissociation with space-based neglect.