Surgical Approaches for Possible Positions of an Olfactory Implant to Stimulate the Olfactory Bulb

INTRODUCTION

Current scientific developments seem to allow for an "olfactory implant" in analogy to cochlear implants. However, the position and surgical approaches for electrical stimulation of the olfactory system are unclear.

METHODS

In a human anatomic cadaver study, we investigated different endoscopic approaches to electrically stimulate the olfactory bulb (OB) based on the following considerations: (1) the stimulating electrode should be close to the OB. (2) The surgical procedure should be as non-invasive and safe as possible and (3) as easy as possible for an experienced ENT surgeon.

RESULTS

In summary, the endoscopic intracranial positioning of the electrode via a widened ostium of the fila olfactoria or a frontal sinus surgery like a Draf IIb procedure is a good option in terms of patients' risk, degree of difficulty for ENT surgeons, and position to the OB. Endoscopic intranasal positioning appeared to be the best option in terms of patient risk and the degree of difficulty for ENT surgeons. Although a bigger approach to the OB using a drill and the combined intranasal endoscopic and external approach enabled a close placement of the electrode to the OB, they do not seem relevant in practice due to their higher invasiveness.

CONCLUSION

The study suggested that an intranasal positioning of a stimulating electrode is possible, with placements beneath the cribriform plate, extra- or intracranially, applying elegant surgical techniques with low or medium risk to the patient and a close placement to OB.

Olfaction in the context of obesity and diabetes: Insights from animal models to humans

The olfactory system is at the crossroad between sensory processing and metabolic sensing. In addition to being the center of detection and identification of food odors, it is a sensor for most of the hormones and nutrients responsible for feeding behavior regulation. The consequences of modifications in body homeostasis, nutrient overload and alteration of this brain network in the pathological condition of food-induced obesity and type 2 diabetes are still not elucidated. The aim of this review was first to use both humans and animal studies to report on the current knowledge of the consequences of obesity and type 2 diabetes on odorant threshold and olfactory perception including identification discrimination and memory. We then discuss how olfactory processing can be modified by an alteration of the metabolic homeostasis of the organism and available elements on pharmacological treatments that regulate olfaction. We focus on data within the olfactory system but also on the interactions between the olfactory system and other brain networks impacted by metabolic diseases.

Diode Laser Turbinate Reduction in Allergic Rhinitis: A Cross-sectional Study

Introduction

Turbinate reduction procedures are recommended for inferior turbinate hypertrophy in allergic rhinitis that fail to respond to medical therapy. Several modalities like turbinectomy, submucosal resection and tissue ablation are available for this purpose. The study aimed to evaluate the effectiveness of diode laser in the treatment of symptomatic inferior turbinate hypertrophy in allergic rhinitis and explore complications related to the procedure.

Methods

This descriptive cross-sectional study was carried out in a tertiary care centre. The study enrolled 60 patients with inferior turbinate hypertrophy with failure of medical therapy. Inferior turbinate reduction was performed under local anaesthesia using diode laser. All the patients were evaluated subjectively for various nasal symptoms using visual analogue score scale preoperatively and during postoperative visit at three months.

Results

The age ranged from 16 to 47 years with median age of 28 years. Twenty nine were male and thirty one were female. There was significant improvement in symptoms like nasal obstruction, nasal discharge, sneezing and decreased sense of smell. Immediate post-operative pain, crusting and persistent nasal discharge were observed as complications of the procedure. However, there was no incidence of mucosal oedema and synechiae formation in our study.

Conclusions

Diode laser turbinate reduction procedure is safe, minimally invasive and effective in relieving the symptoms associated with inferior turbinate hypertrophy in allergic rhinitis resistant to medical therapy and can be performed on a day care basis under local anaesthesia.

Factor analysis of the questionnaire of olfactory disorders in patients with chronic rhinosinusitis

BACKGROUND

Olfactory-specific quality of life (QOL) can be measured using the Questionnaire of Olfactory Disorders-Negative Statements (QOD-NS), which examines various aspects of olfactory dysfunction. It is unknown if certain factors of the QOD-NS differentially impact QOL.

METHODS

Patients with chronic rhinosinusitis (CRS) completed the QOD-NS, 22-item Sino-Nasal Outcome Test (SNOT-22), Medical Outcomes Study Short Form 6-D (SF-6D) health utility measure, and Patient Health Questionnaire-2 (PHQ-2) depression screen. Exploratory factor analysis of the QOD-NS was performed. Associations between QOD-NS factors and other QOL metrics were analyzed before and after endoscopic sinus surgery (ESS).

RESULTS

Outcomes were examined on 132 patients. The QOD-NS contains 4 distinct factors. There was no difference in associations between the different factors and baseline clinical characteristics. ESS had greatest effect size (d) on factors 2 and 4 (d = 0.29 and 0.27, respectively, p < 0.05). Postsurgical changes in the SF-6D and SNOT-22 had the strongest correlation with factor 2 scores (r = 0.29 and 0.34, respectively, p < 0.05), and changes in the PHQ-2 had the strongest correlation to factor 3 (r = 0.24, p < 0.05). Abnormal QOD-NS scores at baseline were associated with effect size increases of 50% to 100% (p < 0.05).

CONCLUSION

The QOD-NS measures 4 distinct factors. Eating-related questions had the greatest improvement after ESS. Health utility and CRS-specific QOL improvement most strongly associated with factor 2, while PHQ-2 changes are most highly associated with factor 3, suggesting a differential impact of the factors of the QOD-NS on varying aspects of QOL.

[Analyses of the olfactory disorder patients' nasal airway resistance before and after endoscopic sinus surgery]

OBJECTIVE

To explore the changes of nasal airway resistance (NAR) and olfactory recovery in patients with olfactory disorder before and after endoscopic sinus surgery.

METHOD

Olfactory functions and NAR in 105 patients with olfactory disorder diagnosed as chronic rhino-sinusitis and/or nasal polyps were measured with five taste test olfactory liquid and anterior rhinomanometry. The patients were grouped according to their olfactory disturbance degree before the surgery and recovery of olfaction after the surgery.

RESULT

Before the surgery, the degree of olfactory disorder were accompanied with varied clinic classifications. Six months after the surgery, the patients whose conditions were less serious before surgery recovered better. Before and after the surgery, significant difference of the nasal airway resistance was found between the three-groups whose olfactory disorder degree were different. After the surgery, no difference was found of the nasal airway resistance between the three-groups whose olfactory disorder degree were different before the surgery and the recovery of olfaction degree were different after the surgery.

CONCLUSION

Endoscopic sinus surgery can decrease the nasal airway resistance of patients with olfactory disorder. We can evaluate the efficacy of endoscopic sinus surgery more objectively combined with nasal airway resistance analysis.

Olfactory changes after endoscopic sinus surgery in patients with chronic rhinosinusitis

OBJECTIVE

To address the controversy over whether olfactory function is improved or not after endoscopic sinus surgery (ESS) in patients with eosinophilic (ECRS) and non-eosinophilic chronic rhinosinusitis (non-ECRS).

METHODS

Between June 2006 and March 2012, 89 adult patients with CRS underwent ESS at Hyogo College of Medicine. There were 55 men and 34 women with a mean age of 53 years old, ranging from 23 to 79 years. The average follow-up period was 10.7 months (3-24) after ESS. Peripheral blood examination, sinonasal CT imaging, and four kinds of olfaction tests [self-administered olfaction test (SAOQ), visual analog scale (VAS), T&T recognition threshold tests (T&T) and intravenous olfaction test using prosultiamine] were performed. We diagnosed ECRS when (i) symptoms of nasal congestion and olfactory disorder, (ii) bilateral chronic rhinosinusitis with nasal polyps (CRSwNPs), (iii) peripheral blood eosinophilia (>7.0%), and (iv) ethmoid sinus dominant opacification in preoperative CT findings (i.e. ethmoid sinuses (E) were more bilaterally occupied than those of maxillary sinuses (M), E/M≥1), were completely fulfilled. We divided the patients into two groups of ECRS (group A) and non-ECRS (group B). Olfaction tests before operation, and at the 3rd, 6th, 12th, and 24th month postoperation were analyzed. The severity and therapeutic evaluation of olfaction were based on criteria of T&T recognition thresholds.

RESULTS

The mean SAOQ and VAS scores showed significant improvement within 6 months after ESS in both group A and group B. In total, the improvement rates were 52.0% (26/50) at 3 months, 58.5% (24/41) at 6 months, 40.5% (15/37) at 12 months, and 41.2% (7/17) at 24 months. The significant improvement of T&T recognition thresholds in group B was maintained for 24 months, whereas those in group A, showing transient improvement, deteriorated after 12 months or more. A significant difference in postoperative T&T recognition between groups A and B was found at the 12th postoperative month. In both A and B, 84% of patients had a response to prosultiamine (positive group) in the preoperative stage. T&T thresholds in the positive group were significantly better that those in the negative groups in the postoperative stage.

CONCLUSION

Olfactory disorders due to ECRS showed transient improvement that deteriorated as time passed after surgery. The olfaction in the non-ECRS patients recovered comparatively well. Postoperative olfactory results were unfavorable in patients without a preoperative reaction to prosultiamine.

[Nasal polyposis and olfactory function: results of the surgical treatment]

INTRODUCTION

Hyposmia is a common cause of functional complaint in patients with nasal polyposis. The aim of the current study was to report the olfactory functional results after sinus surgery.

MATERIALS AND METHODS

A systematic review of the scientific literature was achieved in the Pubmed database.

RESULTS

Overall, 10 series published between 1989 and 2013, involving 959 patients, were selected. The surgery for nasal polyposis, adjuvant medical treatment, may allow olfactory improvement. The results are even better than surgery is as wide as possible and the evolutionary stage is low.

[Fungal dacryocystitis with cacosmia after penetrating keratoplasty--taxonomy and identification of pathogenic fungi based on DNA sequence analysis]

CASE

A 76-year-old woman with a history of penetrating keratoplasty had cacosmia associated with dacryocystitis. Two species of yeast-like fungi were isolated from the contents of her lacrimal sac. Each cultured fungus had a unique, distinctive odor. Althogh treated with an oral antifungal agent and washing of the nasolacrimal duct, the cacosmia was not improved. The continuous dacryocystitis with cacosmia was treated by dacryocystectomy. Two yeast-like fungi were again isolated from the contents of the lacrimal sac. Pathological examination confirmed a diagnosis of fungal dacryocystitis based on the fungal hyphae observed on the excised lacrimal sac wall. The cacosmia promptly disappeared. The fungi isolated from the contents of lacrimal sac were identified by DNA sequencing as Wickerhamomyces anamalus (Pichia anomala-Candida pelliculosa) and Galactomyces geotrichum (Geotrichum candidum).

CONCLUSION

The cause of cacosmia in the present case was fungal dacryocystitis. Antibiotic eye drops and steroid eye drops for the treatment of penetrating keratoplasty can cause atypical fungal presentation in the inconsistently treated lacrimal system and can induce dacryocystitis. Careful usage and consideration is necessary in the long-term use of antibiotics and steroids following corneal transplantation.

Functional endoscopic sinus surgery improved asthma symptoms as well as PEFR and olfaction in patients with nasal polyposis

BACKGROUND

Nasal polyposis is a disease known to be associated with asthma. The management is anti-inflammatory, with topical and oral corticosteroids as the first-line treatment. The effect of surgical treatment on lower airway inflammation has not been sufficiently studied.

AIM

The aim of this study is to investigate the effects of functional endoscopic sinus surgery (FESS) as well as fluticasone proprionate nasal drops (FPND) 400 microg b.i.d. on nasal and lower airway parameters in asthmatics with nasal polyposis.

METHODS

This was a prospective 21-week study of 68 patients with asthma and nasal polyposis, on the benefits of FESS on nasal '(butanol test, subjective olfaction, peak nasal inspiratory flow, congestion, rhinorrhoea, and polyp score)', and on the lower airway parameters (dyspnea, cough, mean daily peak expiratory flow rate (PEFR), and lung function tests). It also included a randomized, double-blind, placebo-controlled 14 weeks phase on FPND.

RESULTS

Functional endoscopic sinus surgery significantly improved mean asthma symptom scores and daily PEFR and all nasal parameters including subjective and objective olfaction tests. This is the first study that shows the benefits of FESS on butanol tests in patients with nasal polyposis. We found no significant difference between topical treatment with FPND or placebo in the nasal or lower airway variables.

CONCLUSION

Functional endoscopic sinus surgery improved nasal and asthma symptoms in patients with nasal polyposis. Functional endoscopic sinus surgery could be considered early in the natural course of nasal polyposis with concomitant asthma, as well as a second-line treatment in nasal polyposis patients with a reduced sense of smell. The potential benefits of FPND 400 microg b.i.d. were probably overshadowed by FESS.

How do children with adenoid hypertrophy smell and taste? Clinical assessment of olfactory function pre- and post-adenoidectomy

OBJECTIVE

To assess the ability to smell and appreciate food in children with adenoid hypertrophy pre and post-adenoidectomy.

METHODS

A total of 35 children with adenoid hypertrophy admitted for adenoidectomy (aged 5-9.3 years, mean age 5.9 years) were tested preoperatively. In a follow up appointment 3 months postoperatively, 28 of them returned and were tested again. The control group consisted of 30 healthy children (aged 5-10.2 years, mean age 6.1 years). Orthonasal olfactory function was assessed psychophysically using "Sniffin sticks" 12 item odor identification test. Retronasal olfaction was assessed using a collection of 12 grocery available powders applied to the oral cavity.

RESULTS

Both orthonasal and retronasal olfactory function of the study group preoperatively were significantly lower in comparison with the results of the control group (p<0.0001). The difference in retronasal olfaction between groups was significantly higher in comparison with the difference in orthonasal odor identification (p=0.048). There is a strong correlation between adenoid hypertrophy grade and retronasal testing results (t=-5.461, p<0.0001), but not with the orthonasal results (t=-1.677, p=0.9). Significant improvement of orthonasal and retronasal olfaction was observed in the follow-up appointment (p<0.0001). Relative increase postoperatively was higher for the retronasal olfaction compared to orthonasal olfaction (p=0.0029).

CONCLUSIONS

Children with adenoid hypertrophy have a reduced ability to smell, with retronasal olfactory function being more affected. Surgery had a greater impact on retronasal olfaction.

Subjective improvement of olfactory function after endoscopic sinus surgery for chronic rhinosinusitis

OBJECTIVE

The purpose of this study was to determine the effect of functional endoscopic sinus surgery (FESS) on subjective olfactory dysfunction in patients with chronic rhinosinusitis. Materials and methods Prospective collection of data on consecutive patients undergoing FESS after failing prolonged medical therapy for chronic rhinosinusitis at a tertiary institution. Patients were asked to grade their olfactory dysfunction from 0 to 10, with 0 representing normal function and 10 complete anosmia. In addition, data such as computed tomography scores, presence or absence of nasal polyps, and the presence or absence of asthma were recorded and analyzed. Patients were followed up to 1 year after surgery.

RESULTS

Data were collected on 178 patients who had sinus surgery over a 2-year period. The average olfactory dysfunction score before surgery was 4.9. This improved to 0.9 at 1 year after surgery (P =.00). Higher computed tomography scores as per Lund and MacKay correlated with higher olfactory dysfunction scores (r = 0.62, P <.01) and greater improvement after surgery (r = 0.82, P <.01). Asthmatics and patients with polyps had higher subjective olfactory dysfunction scores than nonasthmatics and patients without polyps (6.8 and 7.2 v 4.4 and 4.1, respectively). All groups had subjective improvement at 1 year (2.3 and 1.5 v 0.6 and 0.7, respectively; P =.00).

CONCLUSION

Patients with subjective olfactory dysfunction despite appropriate medical management for rhinosinusitis benefit from FESS.

Traitement chirurgical d’une parosmie post-rhinitique

Many people experience some degree of olfactory dysfunction during the acute stages of an upper respiratory infection (URI). The olfactory function usually returns to the preinfection level in few days. In some cases, the dysosmia remains even after the reopening of the air passageways. The olfactory loss after URI associates an hyposmia and odor distortions. We report a rare case of odor distortions after URI treated by surgery.

Anti-inflammatory and surgical therapy of olfactory disorders related to sino-nasal disease

Olfactory loss may be caused by mechanical obstruction or inflammation of the olfactory epithelium due to allergic/non-allergic rhinitis and chronic sinusitis with or without polyps. Treatment of olfactory loss related to sino-nasal disease is possible. Apart from surgical approaches and/or treatment with antibiotics, both systemic and topical steroids are effectively used in the therapy of olfactory loss related to sino-nasal disease. In most cases improvement of olfactory function appears to relate to the anti-inflammatory actions of the steroids used. While some details of therapeutic effect and dose regimen are not clear, systemic steroids are often helpful even in patients without nasal obstruction due to polyps or obvious inflammatory changes.

Olfactory mucosal findings in patients with persistent anosmia after endoscopic sinus surgery

Sixty-three biopsy specimens were obtained from the olfactory region of 15 patients with persistent anosmia and 6 patients with normosmia after sinus surgery. Immunohistochemical examination of all specimens with microtubule-associated protein 5 (MAP5) antisera demonstrated olfactory epithelium in 11 of 18 specimens from normosmic patients and in 12 of 45 samples from anosmic patients. There was a significant difference in the proportion of specimens containing olfactory epithelium between the two groups of patients. In normosmic patients, most of the biopsy samples contained normal-appearing olfactory tissue. However, 2 main patterns of histologic findings were noted in the olfactory mucosa of anosmic patients. First, the olfactory receptor cells were remarkably decreased in number. Second, the orderly arrangement of cells characteristic of normal olfactory epithelium was lost, demonstrating a degenerative appearance. These data suggest that olfactory epithelium can be degenerated even in chronic sinusitis and thereafter extensively replaced with respiratory epithelium, resulting in increased sampling error. Moreover, an unimproved olfactory deficit after sinus surgery may be due to the abnormalities observed at the olfactory epithelium level.

Reversible hyposmia caused by intracranial tumour

Two patients with hyposmia caused by an intracranial tumour recovered olfactory functions after craniotomy. The first case was a 68-year-old male with a tumour metastasized from the lung to the right frontal lobe. The second case was a 75-year-old male with meningioma of the right frontal lobe. Results of T & T olfactometry and venous olfaction tests also indicated suspected central hyposmia. Magnetic resonance imaging (MRI) indicated compression of the frontal lobe by intracranial tumour. Pressure on the olfactory centre located in the frontal lobe produced hyposmia. Decompression of the frontal lobe by craniotomy improved the sense of smell. Therefore, some cases of olfactory disturbance caused by intracranial tumour may be reversible if they are the result of simple compression of the olfactory centre.

Olfactory function after functional endoscopic sinus surgery for chronic sinusitis

One hundred and fifteen patients suffering from chronic sinusitis were observed to analyse the prevalence of olfactory dysfunction and the influence of FESS. Pre-operative questionnaires were supplemented by examination of olfactory thresholds and discrimination. Pre-operatively, 58% of the collective were aware or complained of any olfactory deficit. However, the olfactory tests demonstrated that of the collective 52% were hyposmic and 31% anosmic. Eight per cent of the hyposmic patients presented with an isolated reduction of their ability to discriminate odours. Post-operative improvements were found in 70%. Normosmia was post-operatively achieved in 25% of the hyposmic patients, but only in 5% of the anosmic patients. Olfaction changed to the worse in 8% after FESS. Therefore, the prevalence of olfactory dysfunction in chronic sinusitis is pre-operatively higher, and the rate of improvement is lower than generally assumed. The extent of sinus disease as measured by the degree of intranasal polyposis correlates with olfactory dysfunction. Resections of the middle turbinate may have a negative effect on olfaction, due to damage to the olfactory fila or alteration of the normal aerodynamic pattern within the olfactory cleft. However, this hypothesis is based on a few observations and needs to be verified by further investigations.

[Improvement of olfactory disturbance by endoscopic endonasal surgery for chronic sinusitis]

Olfactory disturbance is the one of the most important symptoms of chronic sinusitis. In the present study, we followed up the postoperative clinical course of olfactory disturbance in patients who underwent surgery for chronic sinusitis. Ninety patients with severe olfactory disturbance or anosmia who underwent endoscopic endonasal surgery for chronic sinusitis with severe olfactory disturbance or anosmia in the preceding three-year period were enrolled in this study. We obtained a high postoperative improvement rate of 78.8%. We compared and examined our cases' postoperative clinical course concerning their olfactory disturbance and various factors in other patients before and after surgery, and obtained the following results: a) Young patients, 30 years old or under at the time of surgery, showed significantly higher rates of improvement than patients who were 50 years old or more at the time of surgery. b) Even though revision surgery was performed, the improvement rates were almost the same as after the initial surgery. c) Although the Alinamin intravenous olfaction test is regarded as an olfaction threshold test, we have seen quite a few cases in which improvement was achieved despite the absence of a preoperative response to the Alinamin intravenous olfaction test. d) There was no clear correlation between the presence of preoperative lesions of the ethmoid sinus and the olfactory cleft and the improvement rates. e) The cases with unsatisfactory results concerning the paranasal sinuses and nasal cavity--those accompanied by postoperative adhesions of the olfactory cleft, recurrence of polyps, etc.,--showed significantly lower improvement rates than cases with satisfactory postoperative results. During surgery in patients with severe olfactory disturbance, we conclude that it is particularly important to adequately clean the lesions of the olfactory cleft and the anterior and posterior ethmoid sinus endoscopically. This ensures leaving clear the olfactory cleft.

[Sense of smell before and after endonasal surgery in chronic sinusitis with polyps]

Although olfactory dysfunction is a common clinical symptom in patients with chronic sinusitis, there are few written reports about the nature, frequency and therapeutic accessibility of the various dysosmias. We analysed pre- and postoperative olfactory function in 78 patients suffering from chronic sinusitis with nasal polyposis. Using a squeeze bottle technique we were able to detect preoperative hyposmia and/or dysfunction of olfactory discrimination in 40% and anosmia in 36%. Only 22% of these patients complained spontaneously of disabilities in smell. Endoscopic surgery improved hyposmias and anosmias in 71% and preoperative dysfunctions of olfactory discrimination in 61%. Postoperative thresholds for 2-phenylethanol and dimethyldisulfide worsened in 9% of all patients. Postoperative olfactory discrimination deteriorated in 11%. Preoperative and postoperative olfactory functions were not predictable in individual cases when nasal polyposis was limited. However, mechanical blockage of the nasal airway was just one of many other pathophysiological factors causing olfactory loss in patients with chronic sinusitis.

Improvement of olfaction following pansinus surgery

This study involves ten consecutive anosmic patients who underwent pansinus surgery and have been followed and tested a minimum of one year. They all had advanced obstructive bilateral nasal polyposis and pansinusitis. Persistent severe loss of smell was one of the chief complaints in all ten patients. The surgery performed included bilateral nasal polypectomies, bilateral sphenoidethmoidectomies and bilateral nasal antral windows. Post-operatively, all were treated with a topical corticosteroid nasal spray. Seven of these patients were asthmatic and three were also aspirin-sensitive, some being cortisone-dependent. The University of Pennsylvania Smell Identification Test (UPSIT) was administered pre-operatively and then at six-month intervals in the post-operative period. Significant initial improvement in smell acuity occurred in seven of the ten patients. One of the seven, however, developed recurrence of the nasal polyposis and again lost his smell capabilities somewhere between the twelfth and eighteenth post-operative months. Another of the patients who did not improve smell sensitivity post-operatively occasionally is aware of fleeting periods of return.

[Olfactory dysfunction: its diagnosis and surgical treatment]

This paper discusses the results of diagnosis and surgical therapy of olfactory dysfunction. It describes a method of qualitative and quantitative olfactometry which involves olfactory investigation by means of an olfactory unit and an olfactometer based on the smelling principle. The olfactory unit and olfactometer permit qualitative and quantitative olfactory measurements. Objective olfactometry was performed in 34 patients using the galvanic skin reflex, olfactory-pupillary reflex (OPR), pneumography, and ECG. The method, developed by the present author for recording pupils responses by means of an optic pupillometer, allows differentiation of olfactory and trigeminal sensitivity during OPR examination. Objective olfactometry demonstrated that olfactory investigation with the aid of a custom built olfactometer gives reliable data. The procedure of endonasal operation of concha nasalis media in the olfactory fissure area in the case of hypertropic rhinitis accompanied by olfactory dysfunction is described.

Olfactory function following late repair of choanal atresia

Results of olfactory function tests (threshold determination and odor identification) in three cases of bilateral and one case of unilateral choanal atresia are reported. All four patients underwent successful repair of choanal atresia at relatively advanced ages (8 to 31 years). Test results showed that patients who had suffered from bilateral atresia had permanent olfactory deficits, while the patient who had suffered from unilateral atresia appeared to have normal olfactory acuity. Although these results should be interpreted with caution due to the small number of cases examined, they suggest the possibility that early sensory exposure might be needed for the normal development of central olfactory functions in analogy to the visual system.