Spotify API 400 Error Only Valid Bearer Authentication Supported

I'm trying to use the spotify api to get data for my top artists and songs. I followed the authorization code examples here https://github.com/spotify/web-api-auth-examples. The authorization works and I can log in and see my basic info and now I'm trying to get my top artists however I get a 400 error: "Only valid bearer authentication supported".

Here's my code for this

    app.get('/get_top_artists', function(req, res) {
  var authString = 'Basic' + new Buffer(client_id + ':' + client_secret).toString('base64')
  var authOptions = {
    url: 'https://api.spotify.com/v1/me/top/artists',
    headers: {
      'Authorization': authString
    }, function(res) {
      console.log(res)
    }
  };

  request.post(authOptions, function(error, response, body) {
    if (!error && response.statusCode === 200) {
      var get_top_artists = body.get_top_artists;
      res.send({
        'get_top_artists': get_top_artists
      });
    }
  });
})

EDIT

    app.get('/get_top_artists', function(req, res) {
  console.log('top artists');

  var authOptions = {
      url: 'https://accounts.spotify.com/api/token',
      form: {
        redirect_uri: redirect_uri,
        grant_type: 'authorization_code'
      },
      headers: {
        'Authorization': 'Basic ' + (new Buffer(client_id + ':' + client_secret).toString('base64'))
      },
      json: true
    };

  request.post(authOptions, function(error, response, body) {
    console.log('request')
    if (!error && response.statusCode === 200) {

        var access_token = body.access_token,
            refresh_token = body.refresh_token;
        var options = {
          url: 'https://api.spotify.com/v1/me/top/artists',
          headers: { 'Authorization': 'Bearer ' + access_token },
          json: true
        };

        // use the access token to access the Spotify Web API
        request.get(options, function(error, response, body) {
          console.log('request 2')
          console.log(body);
        });
     }
  });
})