How Does the Brain Fill-In the Visual World?

REVIEW ARTICLE How Does the Brain Fill-in the Visual World?

ur awareness of the visual environment ings from anaesthetised monkeys show that when comes to us from the pattern of light on filling-in takes place at the blind spot, neural Othe retina. But this pattern is an incom- responses are generated at the retinotopic repre- plete record of the visual scene, because many sentation of the blind spot in primary visual cor- pieces of the scene fall on the blind spot or are tex.3 4 However, the precise mechanism by which obscured by retinal vessels. This loss of informa- perceptual filling-in across the blind spot occurs tion can be worsened by disease-induced retinal is still unknown. The two main theories are that it damage, or when cortical injury following stroke involves lateral propagation of signals from the damages areas of visual cortex corresponding to edge of the blind spot, or is due to remapping of parts of the visual field. Yet healthy people and receptive fields of surrounding neurons into the most patients are largely unaware of this missing blind spot region.5 Geraint Rees or incomplete information. Instead, we see the is a Professor of Cognitive visual scene as though it were complete because Filling-in after prolonged fixation Neurology and Wellcome the brain ‘fills-in’ the missing information. The Filling-in also takes place in normal vision during Senior Clinical Fellow at the neural mechanisms involved in such perceptual prolonged fixation. For example, a figure viewed Institute of Cognitive Neuroscience and the Wellcome ‘filling-in’ can tell us a great deal about normal in the periphery on a bland and featureless back- Trust Centre for Neuroimaging, visual processes, and are also likely to be ground will seem to disappear after a few sec- University College London. involved when parts of the visual system are dam- onds of prolonged fixation, to be replaced by the His research focuses on the neural aged and more extensive filling-in takes place. background (see Figure 2a). This type of filling-in basis of consciousness in health 6 and disease. is known as Troxler fading. A similar but more Filling-in at the blind spot striking effect is seen if the featureless back- Although the blind spot is devoid of photorecep- ground is replaced by a dynamic texture, similar tors and carries no visual information from the to the static on a television set. This dynamic corresponding region in visual space, when we background promotes rapid filling-in of even view the world through one eye, we don’t see a quite salient figures placed on top of the back- blank patch: the visual system fills-in the missing ground, and the resultant effect is described as an information from the surrounding colour or pat- ‘artificial scotoma’ because the figure becomes tern (Figure 1). Behavioural studies in healthy invisible and ‘filled in’ by the textured back- people suggest that filling-in at the blind spot is a ground7 (see Figure 2b). These ‘artificial sco- Rimona Weil rapid, preattentive process that occurs early in the tomas’ may be associated with similar neural is an MRC Clinical Research visual system. For example, if several rings are processes that lead to the filling-in which takes Training Fellow in Cognitive viewed, but with one positioned in the visual field place when targets are stabilised on the retina, as Neurology currently undertaking so its retinal projection lies just around the blind eye movements disrupt the artificial scotoma.8 a PhD at the Wellcome Trust spot, then this particular ring will ‘pop out’ of the Behavioural studies suggest that the filling-in Centre for Neuroimaging at University College London. Her group as it is perceived not as a ring, but as a associated with an artificial scotoma takes place research focuses on the filled-in disc among the other rings that do not in early retinotopic cortex as it is influenced by integration of bottom-up and lie over the blind spot.1 Even an extremely narrow low-level sensory factors such as eccentricity and top-down signals in human border (0.05 deg) surrounding the blind spot, will boundary length of the figure that ‘fills-in’.9,10 This perception in health and disease. generate the appearance of uniform colour fill- is consistent with single cell studies in monkeys11 Correspondence to: ing-in the blind spot,2 consistent with the theory and neuroimaging reports in humans.12,13 Dr Rimona Weil, that such filling-in depends on local processes However, recent work suggests that higher cogni- Wellcome Trust Centre for generated at the edge of the blind spot represen- tive factors may also play a role, as directing spa- Neuroimaging, 12 Queen Square, tation in primary visual cortex. Single cell record- tial attention to the peripheral figure makes it London WC1N 3BG, UK. Tel: +44 20 7833 7472 Fax: +44 20 7813 1420 Email: [email protected]

Figure 1: Examples of perceptual filling-in at the blind spot. Hold the page approximately 15 cm from your face, close your right eye and fixate the cross with your left eye while attending to the horizontal bars. Move the page gently closer and/or further away from you until the green disc falls into the blind spot. When the green disc falls across the blind spot, it will disappear and the space it occupies will be perceptually filled-in by the horizontal bars. Now close your left eye and fixate the cross with your right eye while attending to the pink and yellow target. Again, move the page closer and further away until the target falls into the blind spot. When the yellow disc falls across the blind spot, the appearance will be of a large pink circle, with the yellow disc disappearing and becoming filled-in by the surrounding pink. Similar to example shown in Komatsu, H. Nature Reviews Neuroscience, 2006;7:220-31.

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ABBREVIATED PRESCRIBING INFORMATION (Please consult the Summary of Product Characteristics (SPC) before prescribing.) KEPPRA® film-coated tablets 250 mg, 500 mg, 750 mg, 1000 mg KEPPRA® 100 mg/ml oral solution KEPPRA® 100 mg/ml concentrate for solution for infusion Active Ingredient: Tablets: levetiracetam 250, 500, 750 and 1000 mg. Oral Solution: levetiracetam 100 mg per ml. Infusion: levetiracetam 100 mg per ml. Uses: Monotherapy for partial onset seizures with or without secondary generalisation in patients from 16 years of age with newly diagnosed epilepsy. Adjunctive therapy for partial onset seizures with or without secondary generalisation in adults and children from 4 years of age, for myoclonic seizures in adults and adolescents from 12 years of age with Juvenile Myoclonic Epilepsy and for primary generalised tonic-clonic seizures in adults and adolescents from 12 years of age with Idiopathic Generalised Epilepsy. Infusion: an alternative for patients when oral administration is temporarily not feasible. Dosage and Administration: Oral solution should be diluted prior to use. Infusion: Keppra concentrate must be diluted in at least 100 ml of a compatible diluent and administered intravenously as a 15-minute infusion. Monotherapy (adults and adolescents from 16 years): Recommended starting dose of 250 mg twice daily which should be increased to an initial therapeutic dose of 500 mg twice daily after two weeks. The dose can be further increased by 250 mg twice daily every two weeks depending upon the clinical response. The maximum dose is 1500 mg twice daily. Adjunctive therapy: Adults and adolescents older than 12 years or weighing 50 kg or more: 500 mg twice daily can be increased up to 1500 mg twice daily. Dose changes can be made in 500 mg twice daily increases or decreases every two to four weeks. Elderly: Adjustment of the dose is recommended in patients with compromised renal function. Children aged 4 to 11 years and adolescents (12 to 17 years) of less than 50 kg: 10 mg/kg twice daily, increased up to 30 mg/kg twice daily. Do not exceed increases or decreases of 10 mg/kg twice daily every two weeks. The lowest effective dose should be used. (For full dosage Figure 2: Filling-in after prolonged fixation. (a) Example of Troxler fading.7 Hold the page recommendations see SPC.) Patients with renal impairment: Adjust dose 20cm away from your face, fixate the cross with both eyes open, after a few seconds, the according to creatinine clearance as advised in SPC. Patients with hepatic blue pattern will fade and disappear. impairment: No dose adjustment with mild to moderate hepatic impairment. With severe hepatic impairment (creatinine clearance <70ml/min) a 50% (b) Example of an artificial scotoma. A square figure is placed in the near periphery on the reduction of the daily maintenance dose is recommended, as the creatinine background of dynamic twinkling noise. Participants fixate centrally and the square figure clearance may underestimate the renal insufficiency. Contraindications, gradually fades and disappears. Warnings etc.: Contraindications: Hypersensitivity to levetiracetam, other pyrrolidone derivatives or excipients. Precautions: If discontinuing treatment reduce dose gradually as advised in SPC. Due to its excipients, the oral solution may cause allergic reactions (possibly delayed). Infusion: Keppra concentrate contains 7.196 mg of sodium per vial. To be taken into consideration by patients on a controlled sodium diet. Monitor patients for signs of suicidal ideation and behaviours. Advise patients and carers to seek medical advice should such signs emerge. Interactions: Keppra did not affect serum concentrations of phenytoin, carbamazepine, valproic acid, phenobarbital, lamotrigine, gabapentin or primidone. Drugs excreted by active tubular secretion could reduce the renal clearance of the metabolite. Levetiracetam 1000 mg daily did not affect the pharmacokinetics of oral contraceptives (ethinyl-estradiol and levonorgestrel). Levetiracetam 2000 mg daily did not affect the pharmacokinetics of digoxin and warfarin and prothrombin times were not modified. Pregnancy and lactation: Should not be used during pregnancy unless clearly necessary. Breast-feeding not recommended. Driving, etc: Caution recommended when performing skilled tasks, e.g. driving vehicles or operating machinery. Adverse Effects: Incidence of undesirable effects considered to be at least possibly related in controlled clinical studies: Very common (r10%): asthenia/fatigue, somnolence. Common (between 1%–10%): GI disturbances, anorexia, weight increase, accidental injury, headache, dizziness, hyperkinesia, tremor, ataxia, convulsion, amnesia, balance disorder, disturbances in attention, memory impairment, emotional lability/mood swings, hostility, depression, insomnia, nervousness/ irritability, agitation, personality disorders, thinking abnormal, vertigo, rash, eczema, pruritus, diplopia, vision blurred, myalgia, infection, nasopharyngitis, cough increased, thrombocytopenia. Consult SPC in relation to other side effects. Pharmaceutical Precautions: Tablets: None. Oral solution: Store in original container. After first opening use within 2 Figure 3: Occluded objects (a,b) Visual search with for amodally completed target. months. Infusion: Use immediately after dilution. Legal Category: POM. Identifying the notched circle is harder in (A) as it is amodally completed and perceived as a Marketing Authorisation Numbers: 250 mg x 60 tabs: EU/1/00/146/004. 500 mg circle among the other circles. In (B), the notched circle ‘pops out’. Search display similar to x 60 tabs: EU/1/00/146/010. 750 mg x 60 tabs: EU/1/00/146/017. 1000 mg x 60 those used in15 (c,d) Depth cues to identify occluded objects. The left-hand image (c) is dif- tabs: EU/1/00/146/024. Solution x 300 ml: EU/1/146/027, Infusion (500 mg/5 NHS Cost: ficult to interpret. (d) Adding a blue snakelike occluder helps to define the occluded figures ml) x 10 vials: EU/1/00/146/030. 250 mg x 60 tabs: £29.70. 500 mg 16 x 60 tabs: £52.30. 750 mg x 60 tabs: £89.10. 1000 mg x 60 tabs: £101.10. as uppercase Bs. Similar to example shown in Nakayama et al., 1989. Solution x 300 ml: £71.00, Infusion (500 mg/ 5ml) x 10 vials: £135.00. Name

and Address of PL Holder: UCB Pharma S.A., Allée de la Recherche 60, 14 B-1070 Bruxelles, Belgium. Further information is available from: UCB more likely to ‘fill in’ and disappear. Thus, although filling-in may be Pharma Ltd., 208 Bath Road, Slough, Berkshire, SL1 3WE. Tel: 01753 534655. generated by low-level, early retinotopic neural processes, it is also mod- Fax: 01753 536632. Email: [email protected] Date of Revision: ulated by higher cognitive factors. January 2009

Filling-in behind occluders The world is very cluttered and most objects do not present themselves Adverse events should be reported. Reporting forms and in isolation but are seen at least partially occluded by other objects. Yet information can be found at www.yellowcard.gov.uk Adverse events should also be reported to UCB Pharma Ltd. we do not have the impression of being surrounded by fragmented objects. The brain ‘fills-in’ the missing information and our impression is the familiar one of viewing complete objects. This type of filling-in of objects behind occluders is also known as amodal completion and © 2009 UCB Pharma Ltd. ® Keppra is a registered trade mark seems to occur at slightly later stages of visual processing than the filling- of UCB Pharma Ltd. in of artificial scotomas and the blind spot. The effects of amodal comple- Date of preparation: February 2009. 08KP0188 tion can be seen when subjects are asked to search for a notched circle

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REVIEW ARTICLE

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Baker CI, Peli E, Knouf N, Kanwisher NG. Reorganization of visual processing in isation remain unknown. One possibility is that it arises from disinhibi- macular degeneration. J Neurosci 2005;25:614-18. 29. Dilks DD, Serences JT, Rosenau BJ, Yantis S, McCloskey M. Human adult cortical tion of pre-existing long-range horizontal connections in V1,32 but this reorganization and consequent visual distortion. J Neurosci 27:9585-94. would require connections longer than those known to occur in primate 30. Baker CI, Dilks DD, Peli E, Kanwisher N. Reorganization of visual processing in mac- V1.33 Alternatively, new horizontal connections might be formed.34 A ular degeneration: replication and clues about the role of foveal loss. Vision Res third possibility is that reorganisation occurs due to new or unmasked 2008;48:1910-19. feedback projections from higher visual areas with larger receptive fields 31. Masuda Y, Dumoulin SO, Nakadomari S, Wandell BA. V1 projection zone signals in 35 human macular degeneration depend on task, not stimulus. Cereb Cortex (see also reference 35 for an example). 2008;18:2483-93. 32. Das A, Gilbert CD. Long-range horizontal connections and their role in cortical reor- Conclusion ganization revealed by optical recording of cat primary visual cortex. Nature Perceptual filling-in, in many different forms, plays a critical role in com- 1995;375:780-4. pleting missing information in normal human vision and is also a con- 33. Angelucci A, Levitt JB, Walton EJ, Hupe JM, Bullier J, Lund JS. Circuits for local and global signal integration in primary visual cortex. J Neurosci 2002;22:8633-46. sequence of visual loss. The mechanisms are likely to differ between 34. Darian-Smith C, Gilbert CD. Axonal sprouting accompanies functional reorganiza- the various types of filling-in but may be important in designing treat- tion in adult cat striate cortex. 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